Shouldn't that be entire point of enforcing speed limits. Setting them up in dangerous locations where demographic in general is speeding?

Why isn't the method used to find those places also subject to bias?

Widespread speed enforcement needs to be almost algorithmic in it's spread. And that algorithm would have to be closely watched. Saying "put a speed camera at every stop light" would heavily police urban centers for example, but essentially exclude wealthy communities that sprawl out with miles of stop signs between multi-acre properties