Regulations regarding Braking redundancy on single speed bikes.
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.