Supposedly what he did, making a "terroristic threat," is actually a federal crime. But I agree, it's really stupid that he was arrested for it.
At the same time, I'm kind of torn on things like this. I would assume (I actually haven't read it) that the law is written as it is so they can lock up true terrorists and not give them wiggle room. The problem is that with such straight-forward laws, they can then be applied to people like that who just have poor taste in jokes.
It's kind of like how i feel about other laws like statutory rape. Take an example where a teenager gets charged with sleeping with his younger teenage girlfriend, who he has been with since they were both below age of consent. I think it's beyond dumb that he could go to jail for that. At the same time, I understand why a line has to be drawn in order to charge the true offenders, and that line is age. Otherwise, the true bad guys will have more loopholes to go through.
Hopefully when it comes to trial the judge will throw it out. I seriously doubt he will serve any more jail time than he has prior to making bail, but dumber things have happened.