Steve:
We're generalising here, but quite often you find a real hatred amongst Anarchists (who, like it or not, are represented fairly heavily among the people who break off from protests and turn violent) for the police. The reason the anarchists hate the police is that they view all state power as inherently coercive.
--
While it may be satisfying for some people to rant about "PC" and "human rights", I'm not convinced there's much to it in this case. The reason the police can't just arrest everybody is simply because of the scale - it's just not economical to keep hundreds and hundreds of people in cells. Plus they all have to be processed etc. etc.
The police approach (as they've said before) is to make forays into the crowd and try to take out individual ring-leaders. Personally, I've no idea whether it's actually effective or not but it's certainly more practical.