That assumes that transactions are always processed immediately. This is not the case. Some gas stations, and convenience stores, for example, will wait up to a week before actually charging your card (they frequently just check to see if you have funds available at swipe time). This saves them money on batching charges. As a result it is possible, with a debit card, to run up charges that exceed the available balance. That's why a credit check is required to give out a debit card with a visa / mc symbol on it.

So, even in the US, if you have bad enough credit, you may not be able to get a debit card even though in theory you shouldn't be able to charge more than you have.

reality != theory