He has stated blatant falsities (man is that even a word?) on national TV...
oh wait.. he's a lawyer.
Anyway, Gabe and Tycho have plenty of legal experience and they have lawyer(s) to back them up. They have proven themselves very good at knowing when they are beat and when they aren't. Of all of their legal issues, the only time they were beat is in the American Greetings / Strawberry Shortcake fiasco, and they took the proper measures to avoid a lawsuit of any kind.
No matter how hard Jack tries, nothing can be done to Penny Arcade about what the forumers are posting. The absolute worst case scenario is that they would be ordered to tell who some of the posters are, but the ones sending the letters aren't even trying to hide that to begin with.
The interesting part about the whole thing is that Gabe and Tycho basically said they aren't going to fight the fight... saying that Jack is basically harmless and its better to have an incompetent person fighting against the video game industry than a competent one. This could be because they were advised by their lawyer(s) not to mess with it, as it isn't worth it.