What others have said, and

Apart from the arguments made by you, Sigma and Lateralus, you have to take into account that banning speech doesn't prevent it from being spoken; only pushes it underground.

I would much rather have this type of militant speech in public, where it can be witnessed by everyone, than underground. For reasons that should be obvious.

I have many times quoted Milo Forman on this, and I will do it again: Freedom of speech is there to protect the ideas that make us uncomfortable. The ideas that everyone agrees upon need no protection.

But of course, the internet has made it ultimately impossible to censor any idea - not that governments have stopped trying.