Do you know what your Municipality (or County, or State) permits a homeowner to do with regards to the removal and disposal of asbestos cement shingles?
Do you know if the dumpster was 'labeled' as Hazardeous Waste?
It's quite possible that if his 'actions' were reported, and assuming he was breaking the shingles and illegally disposing them in a dumpste (in quantity), he would have gotten in trouble.
Unfortunately, for a lot of folks, out of sight is out of mind.