See, there's a popular misconception that moderation on social networks and other Web sites is governed by the First Amendment. (For more on this mistaken point of view, please see the comments section of virtually any Washington Post story.) That is not, however, technically correct. The First Amendment defines the relationship between you, as a citizen, and the government. It does not define your relationship between, say, you and a private corporation, or you and the university you attend, or you and your neighbourhood association.
That means this notion we have about radical free speech - this distinctly American framework, that anyone can say anything, more or less, short of screaming "fire" in a theatre or making a "true threat" - does not have to apply to online spaces. Instead, companies like Twitter can make new standards, new frameworks, according to their corporate values and the needs of their users. (Twitter, a longtime holdout here, has recently escalated its attempts to make sure that "differences of opinion do not cross the line into harassment.")
This does not, alas, mean we've seen the last of Chuck Johnson, uber-troll extraordinaire. Banned by Twitter, he can simply take up residence elsewhere - say Facebook, where he purportedly maintains a personal account, or Reddit, where he's promised an upcoming AMA.
But even Reddit, the mainstream Internet's long-time champion of absolute free speech, modified its rules to clearly ban harassment on May 14. There's a growing understanding, it seems, that the standards we use for speech off the Internet are not quite the same as the ones that work on it.