In reply to Alex Pryor:
Normally it's done when an enforcement officer is on site.
When you have several establishments together it's incredibly silly and needs to be banned. It is effectively anticompetitive, as a combination of that rule and a "no return within" rule[1] means it is impossible to shop around between two adjacent premises e.g. two DIY sheds.
TBH, the better way of doing that, if you *really* want to do it, is to charge for parking but refund it when you make a purchase at the store that owns the car park.
[1] Also a stupid rule when car parks are camera controlled, because you can't return for a forgotten item, and it also creates issues for taxi drivers and people dropping off/collecting non-drivers. More sensible is "you may park for a maximum of three hours total in this car park in any calendar day" or somesuch.
Post edited at 12:42