1. The Hitler reference seems to be, as already stated, just a metaphor of extreme contrast between committed enemies, not saying, "Barack Obama is like Adolph Hitler."
2. The harsh part of what he said is that leaders of the political parties should view each other as such enemies that they should not even meet, talk or play golf together. That is the opinion up for criticism, I think.
3. I guess I think of Hank as a cussing, drinking entertainer type, so hearing some "shock-jock" type statements isn't that surpising--although, again, this was pretty benign. This exact kind of exaggerated metaphor is on every morning on local talk shows--no, not just political ones--for attempted humor.
4. At the least, Hank was going public with his dislike of the president, and did so with strong emotion. If Hank's employers, fans, or sponsors don't like anything about him--including that--they have every right to spend their money somewhere else. In that way, it is similar to the Dixie Chicks.