In GeoCaching there are rules that you agree to uphold when you sign on.
Geocaches can't be hidden at 'sensitive' locations(military installations, anything of strategic importance and so on), and must be on publicly accessible land.
It's NOT allowed to 'modify' the area by digging, drilling or whatever in order to hide the cache, without getting permission from the land owner. (moving a couple of rocks is usually considered OK)
Leaving the area as it was before is one of the points, really. Can't make it too easy for others.
Yeah, a lot of caches do break the rules...
(I've found them at bridge foundations, and even once in a working mine... )
I do report them to the admins whenever I find them, and if they don't do anything about it, I alert the property owner or authorities.