Occasionally they will be innocent. Reasonable people may disagree. Some may see freedom to congregate as more important than discouraging the sale of drugs. But more of one means less of the other. One may not rationally blame the cops for checking out hangers-around, and then blame them also for not stopping the drug traffic.
Cities go back and forth between strict enforcement of laws and a laxer treatment.
A popular theory today, which may well be correct, is that you discourage major crime by cracking down on the little stuff. The cops get told to stop the petty gambling on the hood of a car, the beer-drinking in the front yard with a buddy, the standing in clumps in alleys of young males who don't live nearby. To the people in the neighborhood, it can look like sheer harassment. To me, in the police car, it looks like sheer harassment. Yeah, it's the law.