It is one of the instances in which the system fails to model reallity, unfortunately.
Honestly I'm of the opinion that "thrown weapons" should be a common ability, similar to brawl. The point of having martial abilities separate is that common people were forbidden to have access to the weapons (which is part of why knife is under brawl) if the troop wants this as a house rule we can certainly make it such.
Certainly would make sense, as thowing rocks shouldn't be that martial... On the other hand, throwing javelin and axes is questionable for peasants. Knives fall somewhere in between.
I'm torn on whether house ruling it would be good or bad.