As stated by many others, CD's can fly if replaced by another CD, and it's done all the time.
The crux of the matter is how many active CD's are there in the area?
If there are five or more, each CD can easily sit out one contest a year and "just CD".
If there are only one or two CD's in the area, you can't expect them to refrain from entering every contest they work.
If some non-CD's are bothered by the practice, maybe they should upgrade to CD themselves and then just CD and not fly.