I would point out to people that sending threats and insults to public figures that way has been a crime for a very long time in the US and comes up once in a while. I admit I do not know the exact heading it falls under, but has been a "routine" issue and for all comments on free speech that recur, it has remained in force.
To put things into perspective, during the last presidential election there was an incident where a couple of students hung an effigy of Obama in their school, and this lead to a witch hunt where they were found and prosecuted as I recall. We even discussed this here, and there was a decent amount of traffic in both directions on whether or not it was justified.
Telling this guy he can't come to the US as a result is a measured response. Technically the goverment could probablty make a case to extradite him and prosecute him, but that would be overkill under the circumstances.
I don't know the exact laws in question at the moment, but I would imagine a big part of the issue is not so much that it's slander or libel, but that no matter what they do Public Figures are going to invoke so much ire from some people no matter what they do, that if this kind of thing was let go it would be non-stop. Truthfully I think the actions in this case are more resonable than the "effigy incident" (despite it being a "hate crime") largely because public officials (loved or hated) are busy enough and receive enough mail where getting say 10,000 letters from people doing nothing but insulting them would bog things down even further. Filtering mail can be difficult enough as it is. For policies against it to work, there needs to be a strong enough deterrant in place to discourage people.
Criticism in public forums and the like is a bit differant than private communication of this sort. Getting on a forum like this and flaming Obama doesn't actually do anything to affect the function of the goverment. On the other hand if letters get to him (however it works) do nothing but insult him, the time he spent on those is time that could have otherwise been spent running the goverment or dealing with more important letters, or heck even reading more informed and reasoned criticism.
I just got home from vacation a few hours ago and am beat to be honest (despite it being a vacation it wasn't all that relaxing), I can't think well enough to really form a strong opinion, as this is one thing that can be argued both ways with reasonable points on both sides. My gut feeling is basically that even though I'm not an Obama fan, I think discouraging people from writing drunken hate mail to him is a good thing. One way or another he's the president, and has to run the country, I might not like how he does it, but the job still needs to be done without this kind of thing. In the end this amounts to a slap on the wrist because honestly I doubt this guy ever had plans to come to the US to begin with irregardless of what he might say to get his five minutes of fame at the moment.