"Failing to pay the fee doesn't automatically break that contract."
IANAL, but aren't contracts only valid if there is consideration for both parties? If they're still letting him use their cars then the contract is probably legally binding, but if they've already cut him off then he can probably argue that the contract is invalid and he doesn't have to pay off the debt.
No, because you have used their service and owe them money for that service. In the case of zipcar, if the service has not been used, or there are no outstanding charges on the account, automembership should fail gracefully. Cellphones will not cut off your account if you are late paying, but you owe the money for the time service was provided.
IANAL, but aren't contracts only valid if there is consideration for both parties? If they're still letting him use their cars then the contract is probably legally binding, but if they've already cut him off then he can probably argue that the contract is invalid and he doesn't have to pay off the debt.