Slavery is not 'negative reinforcement training'. Indeed, it is possible to be a slave and be rewarded for good work. Slavery is where you have no
choice but to work. If the food reward is substantial enough to be worth a crow's while, then it confers a competitive advantage and crows that refrain from this behaviour will face pressure on
their food sources from the offspring of the well-fed crows; essentially you've artificially raised the carrying capacity of the ecosystem, for those crows that agree to your "bargain". Effectively, for the crows, the choice becomes "pick up cigarettes or starve".
I wouldn't worry about it though. If it ever gets tried on a large scale it'll be a hilarious disaster of unexpected second-order effects that will lead to the program's cancellation long before the whole crow species is subjugated.