I think it is because there are people who drive off without paying, then the staff end up paying for the fuel, which in my opinion is unfair to the staff working at the petrol station.
I see the same thing when I go to some petrol stations too, I just treat it as the staff protecting their wallets, nothing else.
Just remember the staff working at petrol stations work shifts (day and night), whether the temperatue is high or low, whether it is raining or sunshine, exposed to the smell of petrol/diesel, breath in the exhaust from our vehciles.....