Well, freedom of speech does allow this. And some people's definition of "insult" may be someone disagreeing with their views and calling them irrational. It's hard to say.
People also confuse freedom of speech with freedom from facing consequences for what you say. For example, if you are a CEO of Exxon Mobil and you say that the environment exists solely for maximizing profit, then there will probably be a backlash. People are free to call for a boycott of your business. You might get upset, but freedom of speech does not mean you can say what you want, and then not face responsibility for it. This could also happen if a racist had a radio show and consistently uttered racial slurs.
What should never happen though, is violence towards the person(s) you disagree with. If we all did this, civilization would be destroyed.