I find braking with both wheels is often better than braking with just the front. At least when you really need to stop.

I'm not sure if the law varies much from state to state, but I believe William Karsten is correct throughout most of the US. Reading the CA law, it basically requires every bike to have a brake. That the brake must be able to lock up a wheel simply means that the brake must be powerful enough to do so. A pretty minimal requirement, really. Imagine if you had a solo rear brake that was too gutless to lock up the wheel, I suppose could lead to some rather scary moments, not because it can't lock up but because it's probably also a pretty useless brake.