His own inner circle told him he could choose to either resign or face prosecution. He really had no choice at all.
(This link is from www.senate.gov and not from the dayum librul media by the way.)
Link:
http://www.senate.gov/artandhistory/history/common/generic/VP_Spiro_Agnew.htm Nixon's new chief of staff and "crisis manager," General Alexander M. Haig, Jr., was haunted by the specter of a double impeachment of the president and vice president, which could turn the presidency over to congressional Democrats. General Haig therefore took the initiative in forcing Agnew out of office. He instructed Agnew's staff that the president wanted no more speeches like the one in Los Angeles. He further advised that the Justice Department would prosecute Agnew on the charge of failing to record on his income tax returns the cash contributions he had received. Haig assured Agnew's staff that, if the vice president resigned and pleaded guilty on the tax charge, the government would settle the other charges against him and he would serve no jail sentence. But if Agnew continued to fight, "it can and will get nasty and dirty." From this report, Agnew concluded that the president had abandoned him. The vice president even feared for his life, reading into Haig's message: "go quietly—or else." General Haig similarly found Agnew menacing enough to alert Mrs. Haig that should he disappear she "might want to look inside any recently poured concrete bridge pilings in Maryland."
Meanwhile, Agnew's attorneys had entered into plea bargaining with the federal prosecutors. In return for pleading nolo contendere, or no contest, to the tax charge and paying $160,000 in back taxes (with the help of a loan from Frank Sinatra), he would receive a suspended sentence and a $10,000 fine. On October 10, 1973, while Spiro T. Agnew appeared in federal court in Baltimore, his letter of resignation was delivered to Secretary of State Henry Kissinger. Agnew was only the second vice president to resign the office (John C. Calhoun had been the first). Prior to resigning, Agnew paid a last visit to President Nixon, who assured him that what he was doing was best for his family and his country. When he later recalled the president's gaunt appearance, Agnew wrote: "It was hard to believe he was not genuinely sorry about the course of events. Within two days, this consummate actor would be celebrating his appointment of a new Vice-President with never a thought of me." In a way you're right. He wasn't just 'allowed' to resign. He was told to resign.
Do you like my pic of Limbaugh?