I really don't think it matters if it's positive or negative reinforcement. You are making their trained survival dependent on a contraption which accepts trash for their services, that doesn't sound like room and board to you?
No, sounds like a job: do a task, get a reward. You do it, janitors do it. If the crows want...
Once a wild animal is trained to an easy single source of food, it prevents them from naturally diversifying their diet to prevent starvation, removing that natural challenge could be deadly.
This isn't just about doing a task and getting a reward, this is about addiction.