As nowster says, it's an invitation not a right.
As for the amount "set aside" at a pay-at-pump, this catches folk out particularly when on holiday abroad. In France for example, failing to pay (writing a cheque when there are insufficient funds etc) is taken far more seriously than here in the UK, though I believe the legal principles are essentially the same. If you write a bouncy cheque you are in real trouble.
At a pay-at-pump, you can use a debit card or a credit card.
If it is a debit card it is effectively the same as a cheque drawn on your personal bank account, and in France they will lay claim to an amount to cover the maximum potential fuel delivery (Eu100 or similar) in order that you cannot default on the payment. Until the actual transaction clears, that Eu100 in your current account is earmarked for them. If you fill up again before it has cleared then another Eu100 will be earmarked. Folk have discovered that they suddenly have no available funds in their current account until the transactions have cleared.
If you use a credit card, the fuel company is dealing with the credit card company not your personal account, and as such they "know" that they will be paid, consequently only the actual transaction amount gets charged, they don't need to safeguard the potential full Eu100.
Always use a CREDIT card when paying at a pump abroad.