If the original email content is as presented here, it shows seriously poor judgment by a person holding such position of responsibility and power.
A more mature and even-handed approach would have been something like:
"Dear <AOPA>
As you will understand, my current position and membership of AOPA may be perceived as a conflict of interest. Resultingly, I am compelled to resign from my membership of AOPA with immediate effect.
I wish AOPA and its members the best in the future.
Sincerely....."
NOT
"....Please cancel my membership with immediate effect as I no longer want to be associated with AOPA....."
If this is representative of the attitude and response of CASA at the highest levels, no wonder the level of distrust is at such a level.