Playing devil's advocate, I suspect the counter argument is that they are not doubling the charge, merely that you get a substantial discount for prompt payment.
When I see these threads I generally get annoyed because a lot of the time they seem to be people who have inconsiderately parked who are then looking to get out of it due to a technicality such as the white lines of the bay were 2mm too small etc etc. However, this seems to be a genuine...