The main purpose is to encourage users to return them to the correct place in the car park, rather than leaving them in an empty parking space or the roadway, where they are likely to end up colliding with cars.

Little bit of both? Also probably to prevent the temptation of theft. Even though the fee is nominal, it seems to be enough of a detractor?

I don't think "theft" is the correct wording. Rather "taking it with you and not returning."

If it really was about stealing (taking and intentionally keeping), the 1€ or free plastic coin you can get at the counter wouldn't deter you at all.

You can unlock the cart with standard european key tip like this https://image.made-in-china.com/202f0j00GpWUwrYJnBoN/Sn-Euro...

Most homeless and poor people know this trick. It really is just for general populace who are too gullible to know better.

> It really is just for general populace who are too gullible to know better

I know you didn't mean it this way but this is an incredibly funny way of putting it. "The globalists don't want you to know this, but the shopping carts at Publix are free..."

Ill word it like this next time :p

There are also "shopping cart tokens" which you can put on your keychain, and as they are large enough to unlock the cart, but small enough to not get stuck, they can be directly pulled out, and you don't risk to break your key.