They may have nothing better to do but they are still criminals. The latest flash mob behavior is when a large group of young thugs descend on a business all at once. They steal anything they can get their hands on and then quickly leave. There is too much happening at once for the owners or clerks to stop them and they scatter like flies making it difficult for police to catch many or any at all. Since they are usually juveniles it is risky for anyone, police or store owners, to even touch one, much less take them down forcibly.
This actually sounds like the thread on feral dogs. Not much different.