Imagine our disabled driver is Alison Lapper or Frank Williams or someone with loads of money but limited use of arms or hands. The disabled bay is right near the shops but where's the payment machine and how high off the ground is it? Should Frank get into his electric wheelchair, drive to the other side of the car park, up the kerb then ask an able bodied person to reach the slot for him? Ability to pay here means ability to get a 50p coin into the machine, not affordability.
In Bournemouth once we saw massive signs 'the disabled must pay' which I thought they could use as a town motto.