Saw a skit on Jackass a few years back. A character called the 'Ticket Fairy' or something like that who danced along putting money in the meter for people. Then the real parking enforcer came along and had a go at them.
I don't see why you can't. I'm guessing it is just because the charge for clamping is higher than another hour so if you don't allow people to top it up for you then they can take in more money. Stupid really.


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