So the debate really is whether or not you get the controlling skill at level-0 when you directly acquire a sub-knowledge.
A) IF you do, then having Turrets be a default means that Bay Weapons, Ortillery, Screens, SPines, *and* Turrets are all defaults. Turrets gives you Gunner-0 and Gunner-0 gives you all the sub-knowledge, at level-0, associated to that skill.
In this case the addition of "turrets" to the default skills is likely an errata and it's either not supposed to be there (my preference) or it's supposed to be "gunner."
B) IF you don't automatically get the controlling skill at level-0 by acquiring a sub-knowledge directly, then the sub-knowledge skills become theory, nothing more.
This would likely also mean that "turrets" is not really supposed to be a default skill, as how many people would really know the theory behind turret operation, regardless how easy the interface makes it?
Either option (in my ind) leads to removing Turrets from the default skill list.
The option that seems to make the most sense to me is B), that you can acquire theoretical knowledge without knowing anything really practical about it. ("Yes, that's how a Jump drive is supposed to work captain, but I'm a doctor, not an engineer; I can't fix the bloody thing.")