Those kinds of signs are meaningless from a legal standpoint, but stores use them because they do have some positive benefits. A couple of examples:
When customers see a big sign that says “Not responsible for damage caused by carts,” they are more likely to be careful with their own carts, even subconsciously, thus contributing to an orderly parking lot and less risk for everyone.
The store can avoid hassles from people who have suffered minor dents and dings because the vast majority of customers will see the sign and shrug it off, thinking "Oh well, it’s just a small scratch, and besides, it’s not the store’s fault.
It would be great if we could all tape immunity claims to our foreheads, but individuals and businesses can’t absolve themselves from civil liabilities simply by posting a sign.
This doesn’t mean that they ARE responsible for cart damage, but it also doesn’t mean that they AREN’T. Civil liabilities and damages are decided on a case by case basis, taking all sorts of factors into consideration.
Say for instance a parking lot was clogged with carts because a manager had neglected to send an employee out to round them up. Say that having these carts all over the place has created an obstacle course atmosphere, with cars weaving around, parking here and there, not using designated spaces, and in the confusion someone’s grandma gets run over…
You can bet there will be a civil suit, against the driver AND against the store. The court will decide the percentage of fault for each, but the store’s sign will offer them no protection whatsoever.