Most people in the UK would support this action - the police are arresting people who have made hateful, racist or threatening comments. Why is that bad?
The US historically has tended away from laws that apply to people's thoughts or opinions, unless those thoughts or opinions are connected with a tangible crime.
Some whackjob spouting off racist comments isn't considered to be committing a crime as just being an idiot who should be ignored. That being said, if such speech resulted in a riot or a hate crime, or was harassing, the racist would not be free from the consequences of his or her speech, whether the case might ultimately be tried in civil or criminal court (or both).
The Bill of Rights, arguably the most important US legal document, states that "Congress shall make no law … abridging the freedom of speech[.]" That does not mean however, that one is free from the consequences resulting from one's speech.
IANAL, but that's my layman's understanding.
I suppose in the US we are more in the all-or-nothing camp when it comes to free speech (at least idealistically).
Edit: Most people in the UK would support this action - the police are arresting people who have made hateful, racist or threatening comments. Why is that bad?
The concern is that what is perfectly fine one day (and really truly legitimate) can be "bad" the next. Better not to have a category of "bad" speech at all. Otherwise you may be at whim of political fashion.
Or if they are calls to violent action? (Bob tweets asking people to riot, with an added hashtag of a known violent organisation)
I kind of understand why Americans dislike arrests for general hate speech. I understand why some Americans think it's weird that I'm so relaxed about it too.
Basically, part 1 says "you have free expression", while part 2 says "unless that expression could seriously damage people's lives". Works for us.
Viewing from the lens of an another American political issue: separation of church and state, an aspect of freedom of religion. There are people in the US who strongly believe that we should have state sanctioned prayer in schools and feel this doesn't conflict with religious freedom. What they fail to take into account is that at some point their children could be in the school holding prayers for Not_Their_Religion. The point being that separation of church and state is the only policy that protects everyone's religious freedom. Analogously, you can argue that allowing all speech is the only way to protect everyone's freedom of speech.
I really am not an expert on this. This is more of a dorm-room, after-hours student discussion level of understanding on my part and likely more a cultural reflection than a strong reflection of legal foundations, etc.