Windriders have battlefocus, just in case. There might be some that wants to forge scenarios where bikers actually gets off their bikes.
GW wants us to "forge the narrative" right?
But on a more serious and practical note, GML is correct.
Imagine you have an IC with battlefocus and no bike attached to a unit of windriders, if the windriders didn't have battle focus that IC would not be allowed to use it either, but as it now stands that IC can still use it while attached to the windriders.