Human contact is not being minimized at all costs. That would be what they did in Wuhan. Our goal is to flatten the curve so lots of businesses ae still open and we still go grocery shopping and schools have reopened for children.
Officers will wear masks and maybe face shields as well but all they do is shut down the party by ordering the homeowner to shut it down eg off the music. People will leave when they see the police ticketing people who don't.
Nor are we assuming someone at the party is necessarily infected. Most of the time that won't be the case. They have to enforce the law because if they don't everyone who feels like it will have parties and some of them will have Covid.