I really don't get this sentiment, although I see it repeated often online. Hate speech falls under the category of free speech. That is what free speech is, the ability to speak freely regardless of whether it aligns with popular opinion or with those in power. The US has nearly complete free speech, with the exception of libel/slander, perjury, incitement of immediate violence, and a few other areas.
I certainly do not agree with Milo or his tactics. I am no expert in the law, but I would be curious to know if there is grounds to go after him for harassment. Regardless though, free speech is absolutely essential for society to function and progress. This includes the freedom for hate speech to exist simply because attempting to regulate such a thing gives more power to those who control the government. The law may seem favorable when you have someone like Obama in office, but then imagine just how screwed up things could become if Trump and a Republican Congress were able to regulate speech and amend the law.
Challenge speech with more free speech. Winning the hearts and minds of others is far better than silencing those you disagree with, as they will just go more underground and become radicalized. Personally, I would rather know who is who from the get-go.