too was considering writing a letter to the ARC - though my tack was going to be slightly different. I was thinking of writing something along the lines of someone who leaves and joins a different church could be disfellowshipped as well.
It's not disfellowhipping. It's disassociation by one's actions. It might seem a trivial difference but it's there so the WTS does not face any claims of preventing freedom of worship. It's the same reason as why military service is disassociation by one's actions - to prevent the WTS from facing issues with the government for stopping people performing military service.
Typical WT legalese - claim anyone is free to join the military or another church without disciplinary action from the congregation whilst putting a clause in so that the net result in terms of shunning is the same and turn the responsibility for this back onto the person.