While the conservatives on the Supreme Court (joined by Stevens) argued this did not violate the 4th amendment and won due to numbers, the most liberal members in the dissent, argued that this did violate the 4th amendment.
So, although this did become law, it is still entirely valid to agree with these liberal Justices that this violates the 4th amendment.
From the link I posted above:
Mr. JUSTICE MARSHALL, with whom Mr. JUSTICE BRENNAN joins, dissenting.
The Court concludes that because individuals have no actual or legitimate expectation of privacy in information they voluntarily relinquish to telephone companies, the use of pen registers by government agents is immune from Fourth Amendment scrutiny. Since I remain convinced that constitutional protections are not abrogated whenever a person apprises another of facts valuable in criminal investigations, see, e. g., United States v. White, 401 U.S. 745, 786 -790 (1971) (Harlan, J., dissenting); id., at 795-796 (MARSHALL, J., dissenting); California Bankers Assn. v. Shultz, 416 U.S. 21, 95 -96 (1974) (MARSHALL, J., dissenting); United States v. Miller, 425 U.S. 435, 455 -456 (1976) (MARSHALL, J., dissenting), I respectfully dissent.
Applying the standards set forth in Katz v. United States, 389 U.S. 347, 361 (1967) (Harlan, J., concurring), the Court first determines that telephone subscribers have no subjective expectations of privacy concerning the numbers they dial. To reach this conclusion, the Court posits that individuals somehow infer from the long-distance listings on their phone bills, and from the cryptic assurances of "help" in tracing obscene [442 U.S. 735, 749] calls included in "most" phone books, that pen registers are regularly used for recording local calls. See ante, at 742-743. But even assuming, as I do not, that individuals "typically know" that a phone company monitors calls for internal reasons, ante, at 743, 1 it does not follow that they expect this information to be made available to the public in general or the government in particular. Privacy is not a discrete commodity, possessed absolutely or not at all. Those who disclose certain facts to a bank or phone company for a limited business purpose need not assume that this information will be released to other persons for other purposes. See California Bankers Assn. v. Shultz, supra, at 95-96 (MARSHALL, J., dissenting).
The crux of the Court's holding, however, is that whatever expectation of privacy petitioner may in fact have entertained regarding his calls, it is not one "society is prepared to recognize as `reasonable.'" Ante, at 743. In so ruling, the Court determines that individuals who convey information to third parties have "assumed the risk" of disclosure to the government. Ante, at 744, 745. This analysis is misconceived in two critical respects.
Implicit in the concept of assumption of risk is some notion of choice. At least in the third-party consensual surveillance cases, which first incorporated risk analysis into Fourth Amendment doctrine, the defendant presumably had exercised some discretion in deciding who should enjoy his confidential communications. See, e. g., Lopez v. United States, 373 U.S. 427, 439 (1963); Hoffa v. United States, 385 U.S. 293, 302 -303 (1966); United States v. White, supra, at 751-752 [442 U.S. 735, 750] (plurality opinion). By contrast here, unless a person is prepared to forgo use of what for many has become a personal or professional necessity, he cannot help but accept the risk of surveillance. Cf. Lopez v. United States, supra, at 465-466 (BRENNAN, J., dissenting). It is idle to speak of "assuming" risks in contexts where, as a practical mater, individuals have no realistic alternative.
More fundamentally, to make risk analysis dispositive in assessing the reasonableness of privacy expectations would allow the government to define the scope of Fourth Amendment protections. For example, law enforcement officials, simply by announcing their intent to monitor the content of random samples of first-class mail or private phone conversations, could put the public on notice of the risks they would thereafter assume in such communications. See Amsterdam, Perspectives on the Fourth Amendment, 58 Minn. L. Rev. 349, 384, 407 (1974). Yet, although acknowledging this implication of its analysis, the Court is willing to concede only that, in some circumstances, a further "normative inquiry would be proper." Ante, at 740-741, n. 5. No meaningful effort is made to explain what those circumstances might be, or why this case is not among them.
In my view, whether privacy expectations are legitimate within the meaning of Katz depends not on the risks an individual can be presumed to accept when imparting information to third parties, but on the risks he should be forced to assume in a free and open society. By its terms, the constitutional prohibition of unreasonable searches and seizures assigns to the judiciary some prescriptive responsibility. As Mr. Justice Harlan, who formulated the standard the Court applies today, himself recognized: "[s]ince it is the task of the law to form and project, as well as mirror and reflect, we should not . . . merely recite . . . risks without examining the desirability of saddling them upon society." United States v. White, supra, at 786 (dissenting opinion). In making this [442 U.S. 735, 751] assessment, courts must evaluate the "intrinsic character" of investigative practices with reference to the basic values underlying the Fourth Amendment. California Bankers Assn. v. Shultz, 416 U.S., at 95 (MARSHALL, J., dissenting). And for those "extensive intrusions that significantly jeopardize [individuals'] sense of security . . ., more than self-restraint by law enforcement officials is required." United States v. White, 401 U.S., at 786 (Harlan, J., dissenting).
The use of pen registers, I believe, constitutes such an extensive intrusion. To hold otherwise ignores the vital role telephonic communication plays in our personal and professional relationships, see Katz v. United States, 389 U.S., at 352 , as well as the First and Fourth Amendment interests implicated by unfettered official surveillance. Privacy in placing calls is of value not only to those engaged in criminal activity. The prospect of unregulated governmental monitoring will undoubtedly prove disturbing even to those with nothing illicit to hide. Many individuals, including members of unpopular political organizations or journalists with confidential sources, may legitimately wish to avoid disclosure of their personal contacts. See NAACP v. Alabama, 357 U.S. 449, 463 (1958); Branzburg v. Hayes, 408 U.S. 665, 695 (1972); id., at 728-734 (STEWART, J., dissenting). Permitting governmental access to telephone records on less than probable cause may thus impede certain forms of political affiliation and journalistic endeavor that are the hallmark of a truly free society. Particularly given the Government's previous reliance on warrantless telephonic surveillance to trace reporters' sources and monitor protected political activity, 2 I am unwilling to insulate use of pen registers from independent judicial review. [442 U.S. 735, 752]
Just as one who enters a public telephone booth is "entitled to assume that the words he utters into the mouthpiece will not be broadcast to the world," Katz v. United States, supra, at 352, so too, he should be entitled to assume that the numbers he dials in the privacy of his home will be recorded, if at all, solely for the phone company's business purposes. Accordingly, I would require law enforcement officials to obtain a warrant before they enlist telephone companies to secure information otherwise beyond the government's reach.
[ Footnote 1 ] Lacking the Court's apparently exhaustive knowledge of this Nation's telephone books and the reading habits of telephone subscribers, see ante, at 742-743, I decline to assume general public awareness of how obscene phone calls are traced. Nor am I persuaded that the scope of Fourth Amendment protection should turn on the concededly "esoteric functions" of pen registers in corporate billing, ante, at 742, functions with which subscribers are unlikely to have intimate familiarity.
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