Is It OK To Toss Something In A Random House’s Garbage?

I was just heading out on a walk with Chance, our beagle mix, earlier today when I encountered another woman walking her dog. Well, at that moment, they weren't actually walking-- the dog was, uh, 'taking care of business' right there on the sidewalk. When he was done, the women bagged up the dog's doo-doo-- which was good-- but then tossed the bag of poo in our neighbor's garbage can-- which was at the curb for pick-up today. Can you do that?!?

The bag wasn't very big, obviously, and the garbage was already at the curb, waiting to be picked up by the garbage truck, so tossing the bag of poo in it didn't affect anything.  But, it's not a public garbage can-- my neighbor pays to have their garbage picked up, as we all do. There have been many times I've been out on a walk with Chance, and when he did his business, I promptly bagged up the poo-- but then had to carry it with me until we got home-- even if it was trash day and we passed many other cans on the curb waiting to be picked up.

Am I wrong? Can you just toss something out in somebody else's trash these days?