I think you may have opened up a very large can of worms. At the end of the day anyone can wear what ever they want, tartan included. So long of course that they don't break any rules, either at work, school or actual law. For a Scottish clansman, he or she would most obviously want to wear the tartan of their clan, it would be the same as a military badge. The tartan would make the wearer recognizable as being from Clan "A". But in the same token there may be those whom like the tartan from Clan "A", and chose to wear it as well. There is no harm in that, and certainly not illegal. I wear my clan tartan, and no other. Hope this helps.

Frank