If you didn't know any better you could think the police engineer these things just to make use of those nice camcorders they have.
Strange how when they know Saints are much more active they let everyone have a good old knees up.
As you say, surely easier to keep them in and disperse everyone else. Anyone hanging around is obviously then fair game to get arrested.
In this instance I don't think it would have made much difference to be fair as there were always going to be large numbers of Saints around.