LB, its a little more complicated than that. To "pre authorise" an amount not only requires a working internet connection but a pretty fancy legal agreement between BP and the credit provider as well as some very heavy and expensive security built into the bowser control equipment - and that is also subject to other legal agreements.
First off is banking and privacy laws - the banks absolutely and positively will NOT tell anyone else, electronic systems included, what your credit, debit or account balances are, period. That was the first stumbling block when I was involved in this circa 1987.
OK, so we fix that by pre authorisation - the credit provider says yes or no. Even for that you need legal agreements and strict software protocols to prevent the bowser being used for "fishing" to determine balances and to prevent the amount being accessible to someone with bowser or line access.
But now you want to use your credit card and by definition also your pin - at perhaps 3 am in say Boulia? (eftpos accepted). Your bowser requires a (very) secure control head. The objective being that it should be virtually impossible for the machine to be internally "bugged". it is no secret anymore that your pin is encrypted inside the pinpad so your card details do not exist in an unencrypted state anywhere in the machine. there are again legal agreements about possession and secure maintenance of the machine.
All this isn't simple and it costs money.