Because it's often more convenient to fill up your tank all the way, then pay...instead of taking a guess at how much money you need to fill the tank and coming up short...I guess there's incentives for the station as well...if I had to take a guess I would probably be conservative to avoid the hassle so the gas station misses out on some revenue.
Like kremit said, many of the gas stations here are set up so you can fill it first...not the casheirs fault.
