I think you're right in that it is somewhat against the spirit of the GPL. That said, I'd like to offer another perspective: Dual-licensing with the GPL allows for the open sourcing of programs that would otherwise have been proprietary. I think this benefit greatly outweighs the moral quandary. The potential for forking alone is a huge boon to the open source community.

RMS's thoughts on dual-licensing: https://www.fsf.org/blogs/rms/selling-exceptions