To put it simply, you ran into a very bad CM and a very good Management Supervisor. I can tell you as a former CM that one of the things that always made it so special to work at WDW was the empowerment they give the CM's to add some flair to the otherwise mundane daily routines. It is wonderful to do mystery pin trading with Guests, yet I have only ever seen this done by someone that recognizes the backwards lanyard, vest or otherwise and knows it is the way that CM trades.
I think it is ridiculous that you asked so many times and were treated so rudely, yet I do not truly think that the real issue here is Mystery Trading. I was never once made aware that there was a policy against it, and it was thoroughly enjoyable to place some really rare pins on backwards to see the eyes of a trader light up when they chose something wonderful.
All of my fellow CM's did mystery trading from time to time. Not because we were trying to be rude, but exactly the opposite. I think that CM needed serious reprehension and I hope that is what happened. It is also possible that the Manager was new to that Park or to pin trading knowledge and was just trying to assist you the best way possible.
Taking the Mystery Pin Trading away would leave the predictable and nothing else, and while many of us like seeing exactly what we are trading for (myself included), there are just as many who enjoy the mystery trades. I agree with a former post in which they hinted at the fact that if you can tell the CM is currently doing a mystery trade session, look for the lanyard that is not turned around and head in that direction.
If Disney were to send out a bulletin saying that they were making strict pin policies and revoking all creative freedoms thereof to all of it's massively underpaid CM's, many of which are there just to make some magic for Guests because they have kind hearts and are also Disney fans, it would be a sad, sad day.