"It's incitement to political violence."
Is is a horrible thing to say? Yes. Should he be locked up? No. For one, he probably would be if this really crossed the line. The government, rightly, don't play no games when it comes to our leaders' security.
I'm not defending Nugent's speech. I'm even, for whatever it's worth, condemning it. That sort of talk has no place in public discourse, and by saying things like that he's surrendered any chance to ever be taken seriously on political issues. But it isn't something to throw him or any one else in prison over.
Also, I don't know about you, but I've always heard "suck on a pistol" or the like used as an insult encouraging another to commit suicide. It goes without saying that that's not in any way under any circumstances a legitimate thing to say about any president. Is it inciting political violence, though? I'd have to think not, since I can only imagine the Secret Service and the entire security apparatus errs on the side of caution and is, rightly, more aggressive in going after possible threats than is strictly necessary. The fact that Nugent isn't in prison is a pretty good indication that he shouldn't be.