This is one of the reasons the US was voted the world's fattest developed country:
If walking a few extra steps or avoiding scraping a car (which has happened to me) doesn't motivate you to return your cart, how about this: I overheard a Costco employee who was rounding up shopping carts say, "I don't care if people leave their carts all over the parking lot. They pay me all day to do this!"
That is one of the reasons you pay more at the grocery store for your food than ever before. So put your freakin' cart back!