I've spent a long time on this forum defending Mike Portnoy. And I'll defend him again to the extent of saying that I understand him being upset and, as someone said earlier in this thread, would maybe understand if he had said something in person in the heat of the moment. Posting about it on social media... was probably a little much.
I think it's valuable to look at what Mike said himself about realizing that there's a human being on the other side of your actions.
I would definitely say that the fan who wore the ADTOE shirt could have been a little more sensitive about his wardrobe choices. I think also that this is a bit different from wearing a, say, Octavarium shirt to a DT meet-and-greet today. When that album came out, Mangini was just not in Dream Theater. He was probably aware of them but was not in the state of being a member of the band or having ever been one. When ADTOE came out, though, MP had just left DT. In addition, the title "A Dramatic Turn of Events" could easily be interpreted as a direct reference to his departure. So I think there's some validity to the idea of "maybe pick a different shirt?"
But at the same time, that same nugget of advice should also apply to Mike Portnoy. As has been pointed out in this thread, there was also a human being on the other side of that Facebook post. Given that there is some possibility that the fan made an honest mistake, I think Mike should not have been quite so harsh in his remarks. I, for one, at least, would have had no problem with something more like this: "I know that fans like to wear DT shirts to meet and greets to show their admiration of my work in that band, and I think that's great. But I would prefer it if fans would not wear shirts referring to post-MP DT, because that can make me feel baited or disrespected. I recognize that fans want to show their admiration and can sometimes make an honest mistake by choosing a t-shirt that offends me, but I would appreciate it if they would take this into consideration and not wear these shirts to meet and greets. Thanks." Same basic sentiment, but done in a much less cavalier fashion that does not necessarily come across as spiteful towards the particular fan or to the fanbase in general.
With that in mind, I'm going to have to agree with a suggestion that some people have been making for a while: Mike needs a PR manager to help him manage his social media. I know he is proud of being open with the fans and wouldn't necessarily be eager about the idea of putting what he would see as a barrier between himself and the fanbase, but this would help him avoid having these (practically) monthly controversies. If he just had someone there to tell him "maybe if you phrased it this way instead" or "could you be misinterpreting this?" I think it would help him a lot.
Side note: I know Mike screwed up here, but I have to say I'm not a huge fan of the idea of bothering him about it on his Facebook page. I have no issue with discussing it on a public forum like this one, or even making constructive remarks on his Facebook page, but something about the whole "post a picture of the ADTOE artwork as a comment on an unrelated post" thing rubs me the wrong way. Don't you have more constructive things to do than harassing the guy?