The problem isn't really the rude, inconsiderate asses that will insist on taking liberties with someone's site. The problem really is that most people nowadays lack the intestinal fortitude to take issue with said individuals directly in case they cause conflict. But they are happy to come here anonymously and tell us all how peeved they are...
But, as has already been so eloquently pointed out, who cares if someone walks across your particular piece of a field? If they were using your lav or nicking a brat from the grill as they passed through I think you'd have a case but as it is they are taking nothing and leaving only footprints.