Let katanas delimb the person holding them

[saccharineChampion]

I have considered an explanation which may or may not satisfy you; that the delimbing requires precision, but self-hitting is more like flailing the weapon in your own face.

As for it being inconsistent with other weapons - I'd like to note that actually caring about which limb you're targeting is unique to the katana. I don't think it's too unreasonable to assume that self-hitting a limb effectively makes you self-hit your chest instead, which would have no effect for any other weapon but satisfy why the katana is different.