You can insult anyone, for any reason—and people do all the time, regardless whether the person is a politician or not, a public figure or not, etc..
Sure, morally it might be unjust to insult a politician, or anyone for that matter. Yet the cool part about the United States is: it’s OK!
Although being a politician, and secondarily a public figure, makes the insults as public as the praises so much so that you, the observer, are more aware of them as a result, doesn’t mean that the same thing wouldn’t happen to anyone else.