Server owners oftentimes pay private developer work for them to make unique plugins to help their servers stand out and implement specific ideas that would most likely never done before, or extremely rare; hence it's not just available on the market. While it's a loop-hole, the concept of something being unavailable for the general public as it was created by an owner's own resources, would be cool for it to be publicly but that doesn't take away from the fact that the idea was brought up privately, paid for privately, then you have other server owners joining other peoples' servers, and getting ideas to take away.
Since I can directly speak here, as I made that plugin, no, I do not feel the urge to release it to the public.