Generally the courts in the US have put a very limited threshold on what sort of speech is NOT protected for this very reason. I'm very much in agreement with what the author says. Milo and Spencer preach hate, suppression, harassment, and in some cases violence that shouldn't be allowed.
The problem is how do you decide what is and isnt allowed. There is a lot of opportunity for abuse. If you are the Ferguson mayor maybe you argue that BLM is engaged in hate speech and violence and shut them down. Or maybe you shut down people protesting an immigration law, because you call that hate speech. Or how about religious leaders what do you do with them? Is it ok for them to preach hate speech? If not do we shut down all catholic churches because they preach intolerance to gays?