This sort of things happens occasionally. I don't think it's by design, rather sloppiness.

I recently wanted to use the recent Amex promo code. Conditions clearly state that in Europe (only), code-shares are OK. Indeed it worked for a number of destinations, including the two closest to mine. But not for mine. System would just tell me that "there is no longer any fare" associated with the promo code.
I phoned and the agent was able to issue my ticket, after an hour or so on the phone, talking to fares and to his supervisor. I checked and the code was indeed used. (Not long ago I had a similar situation and I got the fare under the code but afterwards the code remained available; no, I did not use it a second time...)