Most businesses lock the dumpsters because trash service is expensive, and if you don't lock them people will pull up with a pickup bed full of trash and fill them up.
You've never had to repeatedly clean trash slurry off of a concrete slab because junkies are terrible people who have no manners. If people could be trusted to not redistribute the trash across the land I wouldn't mind so much
Enforcing that would take a lot more money than a padlock.
A better idea would be to charge businesses for the downstream costs of externalities like waste. Make them self-enforce by making it more expensive to dump recyclable or reusable materials.
Some stuff is in the garbage because a corporation doesn’t want to devalue the item they’re selling by giving away the same thing for free. Waste is waste. There’s not much difference between cotton garments, and a box of pop tarts, from a resources standpoint.