I'd draw a comparison with my mother and her telling me the colours are not right on the computer. She means 'the monitor screen' but as she's not 'into computers', she hasn't learned all the right terms.
Except that at least 'ninja sword' accurately describes a sword used by a ninja. I can understand it'd be irritable, but unless you want historical japanese weaponry placed on the international school curriculum and taught to everyone, you're probably going to have to put up with 'ninja sword' for a sword that's being used by a ninja.
I imagine there's a proper term for 'police baton' too, but it's a baton being used by a police officer, so it works. (In fact I'm sure it's based on an old japanese weapon, my mate had a ton of that stuff and had two of them things, and he wasn't a big fan of COPS, heh.)