This past Friday we had a game of Capture the Flag in my city's financial district. About 600 people came out... no warning given to the cops, no permission, nothing. Was a good time, our side won 2-0 (go blue!) thanks to some solid defense. We were warned at the start of the game to obey traffic laws (ie, don't jaywalk), because the police response to events like this isn't freaking out and cracking down like you'd expect, or saying "go have some fun" like others would expect. Rather, it's "Sweet, we'll make quota in one night!"
Afterward I went to a parkour jam in a nearby district park with some well placed ledges that lead to the potential for a lot of stunts. After 20 minutes, we noticed a cop car on the street, partially hidden by a wall -- looked like they were watching us, so, as an inquisitive sociologist, I went over to ask them what was up.
As it turns out, they were there for an unrelated reason -- to watch that road because of an illegal left turn onto it out of the entertainment district. Basically the way their trap works is that everyone is drinking in their vehicles, and the illegal turn gives the cops the opportunity to check the vehicle for alcohol, and let them off with a warning if they don't find any. Pretty clever, really.
I asked them about parkour, pointing to my friends in the park behind me climbing, jumping and rolling around. They told me that technically it's illegal under a bylaw that bans games in public areas -- the same law which renders ball-hockey in the street illegal. Then they went on to say that this gives them full authority to break it up and issue tickets if they wanted to, but police policy is to only ever enforce this law if they receive complaints.
Thus, in this one evening, I saw three approaches to laws and law enforcement:
1) Policy designed for monetary gain; if they really wanted to stop the jaywalking, they could've just told us all to go home, but handing out tickets for it helps them make quota, so they let us play.
2) Laws which exist as an excuse to investigate for other laws; by having apparently unreasonable traffic restrictions, they can crack down on drunk driving without invading people's privacy.
3) Laws which exist as an excuse to punish related but unlegislatable behaviour; in the case of both parkour and ballhockey, the issue is "creating a public nuisance," not the specific behaviour being done. However, a nuisance is hard to define objectively, so when it, in the mind of the cops, becomes subjectively a nuisance, they instead use another, easy to define but unreasonable, law.
In all three cases we have laws that aren't actually intended to prevent the behaviour they prohibit, but each does so in a different way... and that was just one evening and discussions I had with three police officers, I'm sure many more examples abound.