As I understand this stalking case, the guy’s facing criminal charges for cyberstalking, which in this case seems to be somewhere between ‘being a relentless jerk’ and ‘there may be actual worms in his head.’ I understand free-speech concerns, but Twitter isn’t actually public space: it’s a private forum where people are permitted to register and participate without paying a fee. And I’m not being pedantic, here: it’s that distinction that allows site moderators to moderate sites*.
Which is not to say that, say, Eugene Volokh is wrong to question the law itself. The defendant seems to have avoided sufficiently explicit and credible threats of violence, at least from a legal point of view (warning: I am not a lawyer); and I favor a high bar for that sort of thing. But I think that it makes more sense for Twitter to seek legal redress against somebody who is deliberately abusing their communications network to harass other people. For that matter, isn’t this sort of thing more properly a case for the civil court system anyway?
*This is, as I can attest personally, a distinction that is lost on a lot of ‘libertarians.’