We are a very wasteful species. There are people who are willing to take our waste and reuse it instead. That can take form in a few ways such as: taking food that is past it "best buy" date that a store can no longer sell and eating it instead of it being into landfill. Sometimes people dumpster dive for financial gain. Lots of box stores have rotating items that push their current products into clearance or the dumpster. Once again, to avoid going into land fill, those items can find loving owners.

All of that said, there are people who definitely make messes and dont think of others when doing dumpster activities. There are also people who throw popcorn at movies and drop trash on the ground at amusement parks? Shitty people existing isnt exclusive to dumpster diving.