They seem to be oblivious that if a waterPROOF surface membrane is applied properly, the drywall is NOT in a wet area. But, trying to talk sense into some people is useless. The only thing I'd consider is to show them the TCNA method approval that allows it. But, as said, it's based on a proper install. You do what you have to, so cbu, while harder and more expensive, is usable underneath any of the surface membrane materials, either painted on or those in sheets. Note, not all sheet membranes support drywall - the TCNA procedure references the manufacturer's recommendations. Kerdi, for one, prefers this, but is equally viable over other backing materials to meet local ammendments to the TCNA procedure. I guess the thousands of installs that are in great shape has no credence...fighting city hall is possible, but sometimes not worth the effort when a viable alternative is available.