You can shout this all you like but you would be wrong. You can remove
the licence if you have permission to do so. For the ath c files there
was permission to do so.
Your understanding isn't quite right. One of many things you may get with
dual licensed code is the right to pick a licence from several choices,
you may also get the right to remove some choices from the recipient.
A work that combines GPL and BSD licensed material is not the same as a
work which says I may choose between two licences. If both licences must
always apply (which is a perfectly possible condition to put in a
licence) then putting such a "both" GPL/BSD licence piece of code into
OpenBSD would require any OpenBSD distributed containing it was GPL
licenced when conveyed, which I am *very* sure is not the intent.
Thus what you appear to be doing by putting the ath5k C code in OpenBSD is
conveying it under the BSD licence (making a choice between the two
offered) and conveying a right for parties down the chain to convey it
under one of the licences only.
And as we've already established the header files are quite different.
Doesn't mean its not somewhat rude but illegal and rude are two very
different things.