They commonly take issue with laws against hate speech in other countries. But, actually, they themselves do have limits on speech. The line in their case is issuing death threats. It's an arbitrary line, both hate speech as in suggesting people of a given class should die, and direct death threats, can lead to people being harmed.

I don't much care about the US's arbitrary opinion on where the line is. And I don't think it's really a slippery slope either. There is a very acceptable range of things that could be reasonably considered banned speech.