Based on your username, I'm going to make the wild assumption that you live in Finland. I'm old enough to remember when we didn't have this system in Sweden, and stray shopping carts was never a problem back then. Anectotal and my memory may be a regional thing, sure, but where I live, most people would just put the carts where they belong because it would break the societal norm if they didn't. And my point is just that this system probably costs more than it pays off for the stores that uses it.