If it's purely for game balance it's one thing but it makes no sense to me at all. Granted I'm not a HEMA expert or anything but by the rules something clunky like a balanced longsword with a crossguard is somehow worse at parrying than something nimble like a morningstar (which is a spiked metal ball at the end of a stick) with no hand protection whatsoever just because of the number of hands you hold the weapon with. That just seems bizarre to me. Especially since a shield is not a requirement for it, you can wield two of them and still parry better while now doing more damage than any two handed.