They do that all the time at the Army Corp of Engineer campgrounds, but at least they let you know. Some will say just park outside the gate if you get back late and others will say no parking by the gate and you're out of luck.
A possible reason, as I have spoken to more then a few campground owners with strange hours and confirmed by my Uncle who has been a camp host for the last 30 years. People will come in late night, hook up, sleep and leave in the early AM without paying. My uncle, a night owl, would just put a night check in envelope under the wipers so they knew they were busted and wouldn't leave without paying.