Road riders are also not really welcome on fixed gear rides because they're unwilling to hop curbs and scare drivers to keep the flow. It's different cultures and skill sets for sure but framing it as "discipline" is kinda shitty imo.

There's minimum standards for both motorcycles and cars. Here in the UK, two brakes are required on bikes over a certain size (e.g. small kids bikes are exempt) if used on the public roads. Fixies are considered to have a rear brake as you can slow down using the pedals, but they need a front brake as well to be legal.