In two words - Posse Comitatus
The Air Force used to be part of the Army, made their own branch around 1947.
The Navy and the National Guard are exempt from any restrictions doing law enforcement work in CONUS (Continental US). The National Guard is mostly just part time people (there are a few full time Guardsmen).
So, the Marine Corps, being part of the Navy is exempt from Posse Comitatus restrictions.
Pretty easy to check on this. Oh, and the POTUS can use the Army under specific circumstances.
Pretty simple to use Marines, I would, and I'm retired Army myself. Even was in a "high speed, low drag" unit myself.