I didn't borrow it, it was provided to me as a service for consideration. The consideration is the not-insignificant markup on all the food I bought from them.

The practical reality is you will never get 100% participation in cart returning and the store will therefore always need people to run the parking lot. They know this, I know this, everybody knows this. Since this is the reality, you pay for the employee to return your carts whether you politely return them or not. Or did you imagine that the grocery store provides carts for everyone out of their deep-seated sense of altruism?