Our local Flying J started doing this too, but only on the RV lanes or diesel only lanes. If you get gas for the personal auto lanes, the hold it $75 or $125.
The reason that this is done is because they don't know how much fuel you are getting, and they have to make sure you have enough of credit available to cover the amount of fuel that you are going to get.
I see several people mentioned Flying J prices being higher than other gas stations, that is NOT the case of gasoline here. They have they typically have about the cheapest gas prices in the county, even competing with multiple QT's. But Diesel is a little higher at flying J than across the street.