If you use a debit card at a gas station a "hold" of $75 to $100 is placed on your account. The hold can be there for up to 48 hours. They're freezing your money.
True, Station close to my house freezes $50 and will only let you pump $50 worth, if you need more than $50 you have to swipe it again for another $50 hold.
Best way to get around this is to go inside and prepay using your pin number.