Police cars parked at hotels?

I’ve noticed that many hotels have a police car parked in front of them semi-permanently. I think perhaps off-duty cops guard them for extra cash or something. But isn’t that using public property for private gain? What’s the Straight Dope on this?

I know that in some jurisdictions police officers drive the cruisers as their private vehicles, sometimes their only one. Washington, DC, used to have this.
I notice you’re near DC.
In this case, the cop is just parking his car at his second job, which is working security at a hotel.
Another theory is that the hotel has hired the police department’s officers to guard the hotel in an official capacity. Some parts of the hotel are public places, even though they are private property, and this wouldn’t be outside of the bounds of reason.
When I was in college in Durham, NC, I saw uniformed police officers working security in the local theater, wearing their standard uniforms. When I talked to one of them, he mentioned that his department was being compensated $16/hr for his services. I have no idea how much of that he took home, and whether or not these were overtime hours or part of his regular shift.