Once upon a time, there was a young boy who didn't know what katana or shuriken were. When talking about them to others, he would refer to those weapons as ninja swords and throwing stars because he also had no knowledge of samurai and, he knew medieval knights and vikings didn't use katana or shuriken. One day this boy became a teenager and discovered the internet. With the internet he also came upon something called anime. Within a matter of days after these discoveries, the boy shunned both old terms unless speaking with people who were completely out of touch with such things.
This is a true story about me. People learn and adapt. The kids who say 'ninja sword' today grow up to become the teens who know about wakizashi and, katana. Besides, if you don't want to inform them of their mistake then you have nobody to blame for your annoyance but yourself.