They aren't, but because you can send Foo-Bar as fOo-BaR on the wire, someone somewhere depends on it. People don't read the specs, they look at the example data, and decide that's how their program works now.

Postel's Law allows this. A different law might say "if anything is invalid or weird, reject it instantly" and there would be a lot less security bugs. But we also wouldn't have TCP or HTTP.