The recipients of the tickets will be happy, and the owners of the car park won't even know, so there is a net increase in happiness. Therefore, as a utilitarian, I say that it's ok.
Anyway, it's not necessarily even breaking the rules: you pay for the use of one space for a fixed time, so if you leave before your time is up, it's probably within the rules for someone else to use the rest of the time. It's still only one space for the fixed time that's being used. It depends on whether the ticket says "non-transferable".