That's a good point! But your proposed solution is security by obscurity. What if a rogue employee trained a backdoor into your DNN? What if the NSA asked vendors for a master key?

The purpose of this research is to work as a proof of concept. Sure, this iteration needs to cheat slightly to achieve its results. However, looking at the images that aren't just noise, it seems to be possible to construct less specialized images that fool many nets.