If everyone did it then you'd probably have a dedicated person to fetch the carts doing that basically throughout their shift. The store still needs the carts for more shoppers and with everyone putting them in the grass that process ends up taking longer.
Except for particularly busy times, I don't think you'd see major pile ups.
But I generally agree with what you are saying. It's a valuable question to ask "what if everyone did this".
Yeah and if everyone was littering all the time, the city might employ more dustmen. Or they would say screw it, why waste money and time, when the citizens obviously don't want to live in a clean city.
Maybe everyone should just start murdering people that bothered them ;P. That way we'd have less annoying people and more police.