We don't know enough about runes to start slapping them on things like stickers.
Why can weapon runes only be put onto weapons? What even is a weapon? If I grab a chair, break off a leg, and start hitting people with it, can I place a weapon rune on it? If yes, why? If no, why not?
Sure, tradition plays a large part of it—but that tradition has to be rooted in something. There's a reason Thungni said "only put this rune onto axes and hammers, but never a bolt thrower—you need engineering runes for those (also here's an engineering rune that strengthens the crew's bravery by invoking their pride. Don't put it on a banner)", but hell if I know what it is.