I believe (although I stand ready to be corrected) that since civil airports are, by definition, private land, landing without permission from the owners of said land is trespass (unless they have stated previously that unannouced arrivals are acceptable).
Therefore I suppose the rule would be - "Unless you have obtained PPR, or the owners have stated it is acceptable to land without PPR (or it's an emergency), landing is forbidden".
Although I must point out this is all pretty much an educated guess on my part.