A lot of people - My mom for example - Seem to be getting the impression that a 'Flash mob' by definition performs a criminal act. That's unfortunate, because only the 'bad' flash mobs make the news, the ones in Philly for example.
But a flash mob could be anything. Could be breaking out in dance or song, freezing in unison in a bus station, everyone walking backwards, etc - Anything goofy/funny to make people laugh and/or just stare on all 'wtf.'
Yes, naturally nobody's going to prosecute the organizer of a goofy albeit completely legal flash mob, but if you orchestrate a crime, you should be prosecuted for the same! So yes, of course it depends on what the flash mob does.