Kind of mixed feelings
I have kind of mixed feelings about it.
On one hand... well, I don't think that impersonating someone is (or should be) valid freedom of speech.
Critique, ridicule, parody, jokes, etc, OK, I can see how a public personality shouldn't be shielded from those. But the key there is that they make clear which is, basically, the critic's own opinion and interpretation of it. You're free to disagree with him, and even to think he's a whiny retard.
Impersonation? Nope, sorry. Other than causing disproportionate distress, and a bunch of other problems, I don't see what's the public benefit from that.
And by other problems i mean: think, basically of someone "impersonating" you and mortgaging your house. Or submitting some fake data in your name. It's you who have to deal with the resulting crap. Impersonating a politician is exactly the same: any diplomatic or political fallout, well, he gets to have to sort that crap out.
Worse yet, a whole country can get to deal with the resulting crap. Government actions and opinions get whole countries to be weary of each other. Think posting some racist crap while impersonating your favourite company's CEO, and think of the fallout that would cause for the company. Now think the same for a head of state, and the same fallout is now the problem of a whole country.
Basically, I think "Bush is a retard" or "I bet Bush gets a hard-on from starting wars" is, and should be, covered by freedom of speech. But they also make clear that it's _my_ opinion. But, basically, "Hi, I'm Bush. I was diagnosed with Down's Syndrome as a kid. And I think Real Men aren't afraid of a war. We still learn about the Romans, don't we? I never got this pinko-commie-liberal peace obsession." is a whole other matter. It gives an air of official truth to it. More importantly, the whole concept of justice and fairness involves there being two sides to any issue and hearing both; impersonating someone already tries to replace their side of the story too.
On the other hand, well, it seems to me like they do use it more like censorship than anything else. I don't see it justified as impersonation being fundamentally wrong, but rather as having offended his sacrosanct majesty. I would have thought we're a couple of centuries too late for that.