There’s laws an regulations on how loud you can be on a public place at a given time. I’d guess that the police has enough gear to stay just below the limit, so your own speaker setup would have to exceed the limits and police can come and eventually force you to stop playing “young people” music. The method seems to work, as the beach is clean for the families and other people coming at the next morning and additionally the police doesn’t need that many boats and/or personnel on the site, so they can focus on the ‘bad’ part of the crowd more easily.

If you happen to be a young person you’re of course welcome to the beach, but the music evidently calms the crowd and troublemakers move to somewhere else where they’re more manageable (and out of the water). I’d say that’s a decent return for the effort, but it’s obviously sad that this kind of effort is needed in the first place. Everyone should be able to enjoy the beach and not worry about the trash and broken bottles filling the area.