This article talks about how a flash mob got too big and caused some problems, but it also mentions how the LAPD basically wouldn't do anything if the people were just dancing, but if people were causing damage, then they would have to step in, or if people were doing something dangerous, such as an incident with a flash mob that tried to get onto a bridge being partially demolished.
http://abclocal.go.com/kabc/story?section=news/local/los_angeles&id=8370278