The whole point of those dinners is that they are what is called, "roasts". The whole point of a roast is to "roast" someone. Roasting is what Imus, and Colbert were doing to Clinton and Bush. That was the reason they were asked to be there... to roast the "guest of honor" at the dinner (that would be Clinton in Imus' case, and Bush in Colbert's case). "Roasting" is where someone does a whole routine that consists entirely of making fun of the "guest of honor". That's what a "roast" is. That is what they are for.

Personally, I find the whole concept of "roasts" rather distasteful. But people keep having them, and the "roastees" keep showing up for them. And as long as that is something they all agree on, I don't see how anyone can take offense at what happens there.