Beach patrol. Says City of Oceanside and there’s an ocean / pier in the background. So this is a beach parking lot.

Passenger ridin’ dirty with a seat belt violation.

Generally, ATVs and side by sides are street legal, but not highway legal. In California, it looks like they are: https://ohv.parks.ca.gov/?page_id=26294#operate

But it’s the cops, even if it wasn’t legal, who’s going to tell them no?

I imagine it’s used for patrolling the beach.