@pussywillow: Except it’s not just mud and it won’t necessarily repair itself will it? By cutting down saplings/trees etc it can destroy the root system which hold the earth together, encourages good drainage and prevents erosion if it’s done extensively enough. Also, as posted somewhere above, how do the diggers know the area they’re digging in does not contain wildlife which they’re disturbing unnecessarily?
About 10 years ago, a group of young guys with jump bikes started doing exactly this in my local woods using what looked like piles of earth as the foundations of their jumps. Thing was, those piles of earth was a badger sett which they subsequently destroyed. Despite them not riding there anymore, the remnants of the trail is still there and it’s now pretty much a bog.