A coin deposit is a signal that either the carts are being stolen, or that they are not being returned to the cart corral. Both are signs of a bad neighborhood in my area of the country.
This is a true statement, but I suspect the underlying goal is to reduce overhead and pass the savings on to the consumer and/or the bottom line. This is because if carts are stolen they must be replaced, and if they are not returned to the corral a worker must be paid to do it.