Welp, straight to the point, I have been wondering why these cant be traded, I understand that someone might give them for free, but most likely people will pay for them, but the main concern was not letting like people who cant get these at their level get one easily like buying or for free, because that would mean getting a spiritomb, though they would have to look in the courtyard for the SR which in my opinion is much harder than the basements keystone hunting, because you gotta check every tile in a maze with pokemon.
Also, on other note, while i understand that the phione egg shouldnt be traded, because, well its an egg, plus its semi legendary i guess, there are the fossils that can be traded, sold and dropped, which are much harder to get than the keystone (or at least most of them), and contain pokemon much more wanted than spiritomb (Inall honesty i have trained spiritombs in other games and well… yeah xD).
In summary, the keystone is like a fossil, you must make effort to find the SR which i havent even found in like 7 runs, and the pokemon you get isnt as wanted as those from fossils, so making it droppable, or at least tradable would be nice. Thats all i think o3o