Uhm, maybe it's cultural? Where I'm from, no one would dream of putting their cart back in the store. If we started doing that, the store would be able to cut back on the hours of the high school/college kids and the nice young mentally challenged men who work as baggers. Half their job is to round up carts!
Now, when I lived in NE, I put my cart in the corral or pushed it back to the store. Here though, if there's a corral, I'll put it there. If not, especially at the smaller grocery stores, it goes in a cluster with the other carts.
DH was a bagger in high school and college, and it's SOP to leave the cars in the lot at most grocery stores and part of his job was round up carts. He'll even "stack" them sometimes when we're at our grocery store, just so the baggers will have an easier time with their rounding up later.
But take it back in? I'm not giving anybody any reason to cut anyone's hours.