Ok, here is a question, how likely would you be to 'turn a blind eye' to a minor infraction by someone who was helping you. For instance, you've gone to interview some guy who has information on a series of burglaries, and has volunteered to assist the investigation, feeling he has good evidence, and when you get there, you notice he's left out an ashtray with the remains of a spliff. (consider for this example that's a minor offence)
Would you let it pass as he's being a 'good citizen' for volunteering to assist the police, or would it be your duty to nail him too? OR perhaps would you offer a quiet warning instead? Personally I'd let it pass, as the last thing I'd want to do is create a feeling that helping the police would get me analysed for criminal behaviour.
To comment on the previous posts tho, and to commend you, I'd have to say, I remember being a teen and having a 'metal' jacket, demin covered in patches and metal studs, chains and spikes, etc.
Now I'd get stopped and searched a LOT by the police, although they never found anything, because I never had anything, heh. Never carried weapons, drugs etc. Worst I might have had was a couple of tapes with some ZX Spectrum games on, and they never queried that.
Actually once I was stopped and it was obvious he thought he'd hit the jackpot when he turned out my pockets and found a small 'wrap' of foil. Imagine his disappointment when he found it to be a year old stinky Kit Kat wrapper. (yes I was scummy back then and shouldnt emptied my jacket pockets occasionally.)
Back then I felt I was being picked on because of my looks, but now I realise that I could have looked threatening, and also the simple fact that certain groups are more likely to be breaking the law, its a stereotype in a way but it's true.
For instance, if I saw two groups of youths, emos and chavs, and I only had time to go talk to one lot to disperse them, I'd choose the chavs, as they're generally (not always, note) more likely to cause a public disturbance, lets face it, emos are mainly likely to injure themselves.
As for breaking up groups, or dealing with unauthorised protests, sure it's your right to protest, but the police have to be ready at a moment's notice to shut you down if it gets out of hand, and an unauthorised one is more likely to turn ugly.
As for groups of youths hanging around, I'd agree with an earlier poster, its a combination of ' We know you're not doing anything, but we're around, so behave' and breaking up large groups, as no matter what your intent, you can appear threatening to weaker members of the public, and they shouldn't have to avoid the park, say, because it's no longer a friendly place. I realise its not your fault, you're just hanging out with friends, but I understand the cop's take too.
Anyway, TL

R version...
Most of us here at the escapist don't blame the soldiers for their actions, even if we disagree with the invasion of Iraq, right? Can we offer the same feelings to the police too? They're just doing their job and working and risking their lives to protect us and society, and if you don't like the decisions made, look a LOT higher up than the patrol guy.
(Of course this doesn't forgive the occasional power hungry asshole who abuses his position to go around being a dick, but the vast majority in my experience joined the force to help.)