These are very good questions, and have been debated at extensively (as you might imagine for such important questions) by humanity's legal, ethical, and philosophical scholars.

Here's the current state of things with references as to why: https://en.wikipedia.org/wiki/Freedom_of_speech_in_the_Unite... although I have some emotional disagreement with those policies at times, I can't say I'm well read enough on the topic to really have a strong disagreement personally.