Insofar as I'm concerned it is morally acceptable for one person to shun another if the individual knows precisely the conduct at issue and has made a personal choice to shun based on that conduct. In that case individual accountability for shunning is put under an even more intense lens of scrutiny. They can't, for instance, get away with saying "Well So-n-So did something horrible otherwise they'd not be disfellowshipped." Instead the individual has to be comfortable saying, for example, "I'm shunning So-n-So because So-n-So was asking questions that my religion's leaders could not answer for."
Again Marvin, you have a real point. As has been said before on this forum we choose to shun people all the time. I shunned the neighbour who keeps stealing, shun the traffic warden who gives me parking tickets outside my own home.
We do choose to shun people for various lengths of time depending on what they have done (& typically whether an apology is forthcoming) and usually for a limited time span.
To force them to confront the fact that they are shunning you essentially for a thought crime as no matter what we were df for we are not coming back (so the shunning will be possibly for all time ), can only be good.