The only true regulations in the USA are those set by laws of the local jurisdiction.
For instance, no flying in the national parks.
Some cities, such as Los Angeles, have laws forbidding flying within city parks.
If you're going to fly in a city park, check the city's ordinances for any restrictions.
You may get harassed by local cops who don't know the law.
In that case, just say thank you, land the bird and move on.
Basically, public property is fair game.
Happy flying.