I think the reg say that if a fish is hooked in the gill plate or eye and is legal to keep, then you are encouraged to do so.
I have hooked quite a few silvers from the outside in on a 15" to 24" leader. I think (and this is only a theory) that sometimes the fish bites the corky only and misses the hook. When I feel the hit on the corky, I naturally set the hook, which may or may not be in the mouth and if the planets are aligned and the moon is in the seventh house and the hook is lined up right it's fish on. I have no problem keeping a fish hooked in the mouth from the outside in. I believe that's why the regs have that stipulation in the rules.