What is with the $100.00 limit at the fuel pumps?

I believe the retailer limits the amount and here's one reason they might:

When you buy gas, your card is pre-authorized for the "limit" amount. This is because once the gas is pumped they can't get it back if your card doesn't let the charge through. So your card is charged say, $100 until pumping is done and then it's modified to the actual total. If you raise the limit too high, some customers don't have enough balance left on their account to get any gas, even if they only want $5 worth.