If you return the cart, then on one hand you do what's best for the individuals that might have been inconvenienced by your not returning of that cart.
But at the same time you enable the decision makers to keep avoiding introducing the policy that will make the carts be always returned.
So what should you do?
Be the person that is kind to individuals and labeled as "good" by policy makers that are exploiting your morals to keep sub-optimal solution that is cheaper for them?
Or not return it and be the asshole, but make it a little bit more obvious that the current solution isn't really working?
Good and evil are not one number on the axis. Good and evil are separate concepts, and both are multidimensional.
It might be a fake solution in the sense that someone else has the power to solve it in a better way, but does that mean you shouldn't do it?