This is old hat.

As long as police have existed, they have been abusing power to fulfill short term goals. They know very well that none of the charges will stick. But these arrests gave them the time to do one giant, convienient sweep and clear of their city.

It really has nothing to do with it being a rave. This has been done before to rock concerts, grateful dead shows, and probably 6 times a week to hip-hop/rap clubs.