I'm not sure most people realize the term is incorrect. Anyone can use the correct term even if they don't want to go into the technicalities of genetics. I'm convinced many reptile keepers don't know there is a thing called "incomplete dominance", even among those who dare to go into the technicalities.
I believe the problem existes because (as far as i know) BP's don't have any codominant morphs. Snake keepers never had to make the distinction between codominant and incomplete dominant. When someone first used the term codominant it got stuck in people's minds.