The existence of post-paid services seems to have no direct bearing on whether pre-paid services, like Zipcar, should feel entitled to rapidly go ape on your credit rating when the funding source disappears.
It does absolutely have direct bearing on whether "opting not to provide services is the seller's prerogative" as the parent stated, when the services actually are under a formal contract and post-paid. The argument holds up for Zipcar but not any of his examples.
I think you're making the same point I was...
Sorry about that, you're right, I misread the negative leading into the parenthetical
It's effectively a credit system. You owe them money at the end of the month for services rendered since the last bill.