Could it not simply be a level of sensitivity? Not all plants are created the same, after all.
What I've heard is not only dendrobiums, but copper should also be avoided for thin-leaved plants like coelogyne, zygopetalum, etc.
You might be right though, this could be something based upon a few folks who used copper in to great a concentration or too frequently, so damaged the plants, and that got translated into "don't use it on them..."
I found this article from Cornell University that talks mechanism, and it might give us a clue.