This basically ignores my first point: Per charge transaction fees. As a small(ish) company with not much leverage for big contracts with payment processors, you'll want to have a certain minimum amount you charge to people. With your scheme, they'll still have lots of small charges. Then they have to include those charge fees, which in turn makes people complain again.
Bottom line: It's a conscious business decision to not serve customers for whom it's a problem to once pay 20 bucks or so and then draw from that credit.
Edit: Thinking about it, not even Skype works any different. Why should an IaaS?