I have a debit card (VISA) and they accept it. Previously Google did not accept it, only credit cards. I don't know about prepaid but I think the reason you want to use one is why they don't accept them.
So they won't accept it because it is convenient to skin you if your usage runs amok rather than get what you can actually pay and terminate the service there. The more I rub shoulders with the cloud the less I like it.
Anyway, in most cloud services you can allocate a maximum budget each month, so you don't risk spending more that you want.