There's a lot of good teachers that aren't proficient with firearms; and that's OK. Not everyone can have the right mindset for it.This! Teaching should not be a jobs program for people too fragile for the world. If we can't trust you with a gun, why in God's name do we trust you with a room full of children.
There's more to being proficient with firearms in combat than just the three rules and occasional range days. It's not that easy.
I'd rather have the unskilled say "no thanks" rather than force them to carry because... reasons. It's a tool, not a magic talisman. And like any tool you need to be proficient and, let's be honest, not everyone can be.