Eh, 'cause otherwise people will have to exchange money for coins, sometimes people don't have notes to exchange, etc... Seems like retaining the token is incentive enough to return the cart....
what I don't get is when there's multiple rows of carts, people often put their cart in the row that is already the longest, instead of putting it in the shortest one to balance it out
I'll also go fetch a couple on the way if I see some hanging out. It's not because I'm trying to make the cart wrangler's job easier, but because it's not orderly and it bothers me. I have to consciously limit myself or I'd end up patrolling the whole lot.