This is reasonable I think also. If it was longer than 15 minutes everyone would pay just befpre the hour was up and could possibly stay in the airport for another 20-25 minutes. If you spread this out over let say 25% of the cars in a day in the parking it is a massive revenue loss.

One good thing about the airport parking is that if you are there less than 15 minutes it is free. I have gotten lucky a few times with this :)

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"Diplomacy is the art of saying 'Nice doggie' until you can find a rock - Will Rogers"