We use a mat all the time, goes a long way in keeping the interior of the camper clean. Only one campground I can remember we went that had a mat restriction... and all the campers were using mats.
Other campgrounds have pretty obvious spots to setup with room for a mat.
Local state park is all grass, is pretty much booked for the entire season, and there is little problem with mats killing grass. The only exception to this is when people stay for weeks at a time, but that is almost always in certain sections and is sort of the long term stay area anyways.
I do agree that if someone is going to stay long term, thought should be given to how they affect the campsite. Most campgrounds around me have seasonal areas, which usually have spots to park the camper and/or mats.