When you pay at the pump they don't debit your account for the total, it usually just drafts $1 to verify that the account is open and carries a balance. At a later date, the transaction actually goes through for the full amount at which time either the balance gets paid out by your bank, or there isn't enough for the purchase at the Gas Station lists you for collections and probably adds on return fees.
I'm not exactally sure why they don't just draft the account for the full purchase, but I believe it has something to do with not entering a pin# or signing a reciept when you pay at the pump. Anyway, the gas station trys to limit there potential for a large dollar amount loss by putting a $50 limit on their pay at the pump. I can imagin you'd be more upset if you sold someone $150 worth of something and the payment returned.
Also, if your card is stolen

, someone can go to the gas station and make purchase without have to enter a pin, show ID, or sign a receipt. So by having it set up so pay at the pump doesn't hit your account for a few days gives you some time to realize your card is missing and report it stolen before some guys takes a bunch of $'s out of your account.