It's probably bad parenting, but I tell my daughter that people who litter are bad people. I can probably put it better, but she's young, simple is good, and so litterers=bad people. I honestly think that to essentially be true, because if you litter, you've essentially internally rationalized your entitlement to make your shit someone else's problem. Right there with people who don't put their carts back.

That being said, I do also say to her that sometimes the wind will carry trash from a receptacle, and that sometimes folks have difficulty ambulating, and so there are exceptions.