Supermarkets, like most other businesses, don't want to use parkingeye and their ilk but they also want their own customers using the car park and it not being a free car park for other businesses. Costs of things like relaying tarmac or repainting the stripes, making sure there are enough spaces for customers and dealing with any incidents means they pretty much begrudgingly have to
For example I go to the local post office and there's tons of pot holes and people moan on fb about them. The car park actually belongs to the pub there. which is having to pay for steady stream of school traffic (gridlocked after 3pm), shoppers and takeaway collectors, hence they are bringing it in as none of that benefits the pub, for which they pay to maintain the car park.
You get the same at the old cineworld car pack in boro, and the Halfords / b&m ones at Stockton - which are actually two different car perks and even walking between the two can get you a ticket