Many just don't care, it's all about them, they are lechers and thieves. These are the ones who will rip up churchyards, cemetaries, no respect for anyones property.
The best thing you can do is fill in their mess as best as you can, if anyone is around, explain to them that the shit stain who did that is NOT a representation of the entire metal detecting community, that most of us do care, (which you are actually proving by fixing this guys mess), apologize for the mess HE left, and go about your business.
If you happen to see an individual do this, as said before, they might not know, approach them and be polite and point out to them, hey, its kind of a code of conduct that you fill your holes, explain to them why. If they get snotty, tell you to F off, then follow them to their vehicle to get a plate # grab your phone dial the cops and report them for vandalism. When the cops do show up, make sure you explain to the cop that YOU detect there too and it is NOT you or YOUR group that is doing this, and YOU are not going to stand by and allow someone elses irresponsibility get park shut down on you.... which is why you are reporting the dirt bag.
Every so often I will grab a trash bag and walk the beach for half an hour or so picking up trash, what I do is, when I enter the park, Ill ask the ranger if it is ok for me to do that, of course they are going to say, sure, and probably ask you why, you then say, well I was metal detecting out here earlier and seen a lot of trash around, this looks bad on all of us so i want to take a little time to clean up the place a bit, kind of like a 'thank you' for allowing me to detect here. This is my community too and I need to do my share to help keeping it clean and safe. Doing something like this will earn a lot of street cred with the park folks as to letting you do your activities. To be honest, when the tides too high, or im not in the mood to really detect (yes it happens sometimes) or just scouting out the area for a planned MD trip later in the week, this is great exercise too, which we all can use.
Aaron