You could say you create a paid job by not returning your cart. The downsides I see is that free wheeling carts can cause accidents (eg during wind) and create annoyances such as blocking parking spaces. We could probably find some more arguments on both sides. Personally, I consider the downsides to outweigh the pros, which is why I consider it to be the right thing to return it.
On the other hand, it can be convenient to grab a loose cart that's near your parking space, instead of having to un-jam a cart from the 50 that are stacked inside each other at the front of the store.
The relevant element to me is less the rational construct behind my actions but what the social contract and environment expects of me. I can violate those rules but then have to live with the consequences. In cases such as this I prefer peace over conflict. It’s a small price to pay for me to return the cart and what I gain is not to cause unhappiness in those around me and thus in myself. This is reason enough for me to do it. In other cases, I might want to demonstrate that I do not agree, but I try to be conscious in what fights I want to invest my energy in.