According to [1] French law means they can't store credit card information or directly debit bank accounts. So I can see why a prepay account would appeal to them.
No, he meant that it makes it automated to add money to the prepaid from the CB when some conditions are reached.
It is indeed just a simple process to register the card at the bank and configure the minimum amount before adding money.
Prepaid doesn't mean you have to deal with credits, though. My phone is prepaid, but the carrier still lists the price in real currency, not fake money.
[1] http://wiki.gandi.net/en/billing/faq/direct-debit