Regulations regarding Braking redundancy on single speed bikes. Recently there have been an increase in the number of fixed gear bikes in my (U.S.) city.  Increasingly they tend to have one or zero brake levers, justified by their ability to stop with pedal power alone.

Even more recently, I've noticed even larger numbers of single speed (non-fixed gear) bikes  --- outnumbering the fixed gear bikes.  And they often have only a single brake lever (for simplicity or fashion?).

I understand the justification for having only a single brake on the fixed gear bikes because their braking redundancy lies within the drivetrain.

I do not understand why this is acceptable on single speed bikes.  Is there something I am missing or is that indeed ridiculously dangerous?

Are manufacturers allowed to sell bicycles with non-redundant brakes?  Is this regulated in the U.S. or anywhere else? This seems like a recipe for disaster (having experienced brake failure on my bike).
No, you're not missing something, it is unsafe to have less than two brakes. If you only have one brake and it fails, you're going to have a bad time. Mostly, it's just cool to have one brake, or even zero lever brakes on your fixie. This style is probably just bleeding over into single speed bikes as well.

The law in most states here in the U.S. only requires you to have one brake, and even still there are some rebellious people that challenge how the law defines a brake and how many you should have. There have been court cases in my town that have tried to define whether or not a fixed gear system counts as the one brake, but the judge usually wins these arguments. Many European countries require road-going bikes to have a brake on each wheel.

Manufacturers that sell bikes with only one brake could very well be at risk of being entangled in a legal battle if someone is injured when that one brake fails.