If a bunch of Russians in St. Petersburg pretend to be Americans and post on Facebook or send out tweets in support of Jill Stein, that doesn't mean that Stein has accepted their help.
Nothing in the linked article suggests any illegal conduct by Stein.
She could nevertheless be subpoenaed if she herself had done nothing wrong but there was reason to believe she might have useful information, like a bystander who sees an automobile accident. The linked article doesn't give any indication of that, either. If Stein knows nothing about Russian support for her campaign except what she reads in the newspapers, then there's no valid basis for a subpoena.
If a Russian agent called her up and outlined a scheme by which illegal campaign contributions could be funneled to her, and she immediately turned him down, then she's done nothing wrong but could be subpoenaed to describe the offer. Again, though, nothing in the linked article gives any hint of anything like that.
The bottom line is that a subpoena should not be used for harassment of political opponents.
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