A property owner that allows the public onto their property needs to have insurance against claims that members of the public were injured by the property owner. Such claims can arise out of accidents that happen in the normal course of walking. The claim process itself is expensive to use for settling claims, regardless of the outcome, therefore the insurance premiums are significant.

It is cheaper therefore, to only admit into the parking lot the cars that belong to the customers of the property owner, because there will be less people who could press claims against the owner. When the owner's business is closed, it behooves the owner to enclose the parking lot to keep out people who aren't customers.

I suggest that non-free parking that belongs to a parking company would be preferable, on the above reasoning. Plus fewer spaces would be needed since the company would have no reason to make you move your car if you wanted to patronize a second business.

I patronize a bar where I have to find on-street parking most nights. It is across the street from a supermarket that has a mostly-empty parking lot. I understand why the bar patrons are not permitted to use the supermarket parking lot, and I believe it were best that that parking lot was instead a paid lot open to all drivers. I think about this as I walk to my car.