Great point. Abs does not guarantee better stopping times.
It is however, the platform that all collision mitigation system are based on. So, when your tractor applies more air to the brakes than you ever would - because it thought that parked tractor in the field over there was in front of you while you were going around a bend... Well, that abs system better be working properly, elsewise you'll have one of them jacknife situations and end up in the culvert!
Anywho, I get that your point is the light doesn't affect stopping performance, only the systems other components do.
I guess the non working light violation is just to target the chuckleheads who "fix" the fault by unplugging them.