Police, at least around here, tend to detain pretty much anyone doing anything. Sometimes they over react a bit or arrest the wrong person. A man caught a guy breaking into his house so he asked what he was doing. The burglar proceeded to beat the man. When the police arrived the burglar said he was just walking down the road and the home owner said to come to his house. When he got there the home owner tried to punch him so he defended himself. The police arrested the home owner and let the burglar go free. While the home owner was in jail that night, the burglar returned and started cleaning the house out. A neighbor called in and he was arrested and the home owner finally set free. It was in the news and the officers said they were sorry but they were just doing their job as best they could. Stories like this happen all the time.
I guess from an officer's perspective it might be hard to distinguish what's going on, especially at night, and they can't really trust anything anyone says. They're pretty much always at risk of being injured or killed, so you can't hold much against them. There are times though where they will flip out and do something crazy such as shooting an innocent person, but those are few and far between.