In Chicago, it's done to keep the alleys clean. There's a long history of people digging through dumpsters and making messes, providing homes and hospitality for rats. A previous mayor had a real phobia about rats, and made locking up dumpsters a pretty serious rule.
> I suspect the lock is more about keeping other people's trash out than keeping theirs in.
There are stories about stores/restaurants calling cops when they catch you "stealing" "their" food from a dumpster. Essentially people taking food from dumpsters are treated like overgrown rats... Because https://youtu.be/P7J384IMuMM
While I agree with you, it's pretty easy to claim ownership of an item from your store that you put into your trashcan on your property that you then hire someone to come get.
https://c2.staticflickr.com/6/5244/5362943269_f1d08c480c_b.j...