And to forestall arguments about that being someone's property, trash on the street is considered public property in many (most?) jurisdictions, which is why the police don't need a warrant to go through it.
Edit: and there are ways that this gets slimy too. I'm not debating the maral ramifications of this, just whether it fits the common definition of stealing. It's less theft and more more someone looking for people's small mistakes and making a living my making them very costly for those people. Morally bankrupt, but not illegal. Like the person who drives around looking for ADA compliance issues with businesses so they can sue them, not to make the situation better but just to extract money. Not illegal, but pretty despicable when done for personal gain.