I would like to point out though that you don't have to agree to not respond. That's their rule, not yours, and basically condemning you and then asking you not to respond as if somehow they were the victim is cowardly. I would absolutely respond in any way I wanted. Knowing full well, of course, that they won't listen so I wouldn't try to save them, but I'd be kind while firmly showing them how cruel and messed up what they're doing is and how monstrous shunning is. I would put the blame firmly back on them.
I don't think silence is good. That's exactly what they want. Jehovah's Witnesses want you to feel shame and slink away. Why? So they don't have to face reality. I'd make them see what they're doing. In fact, that's something that woke me up. And since I've been shunned I have walked right up to family I've seen while out and said hi. It makes them shun me to my face, something that is difficult for them, and I want them to feel that. Why should they be awful and have it easy? Easy is often synonymous with enabling and I won't give tacit approval to that behavior.
That's exactly what I did and it worked perfectly.
When I DA'd my parents sent me an email looking like the OP. I chose not to accept their shunning and I wrote a letter back to them explaining it and telling them, among other things, that I still loved them and that my decision had nothing to do with them. I joined the Awake magazine, in which I highlited the famous phrase "no one should have to choose between religion and family".
I let a few weeks pass, then I started to text them again, phone them and visit them from time to time. There was no reply at the beginning and the welcome was very cold the first few months but they never rejected me when I visited them. I persevered and now, 5 years later, we have a good relationship. I even invited them before Christmas, with the illumited Christmas tree in my house, and everything was fine !
PS. Excuse my English but I'm French :)