30 states have tartans? I did not know that. No offense, but why?
Because they can. The U.S. had one commissioned for the Bicentennial (red, white & blue, of course) -- but it had to be registered under a different name, the St. Andrew's. Each of the Canadian provinces has its tartan as well.
Though I am not aware of a big Scots influence in MI history. (There could be, but I am unaware of it.)