I've found an anon comment on Horse New's article for Ted's firing that I believe describes why IDW would fire him or let him go:
I think people are missing something important here, that show staff has repeatedly made clear: The legal issue with origanal content of third parties in the show (or in this case in the comic). I was on a panel of MA Larson about writing, where he got the usual questions about if he ever checked out any fanworks. His answer was, that he is simply not risking it. He signed a contract, that he would not use an original content of a third party or draw any inspiration from that. To not unconsciously write anything into the show, that belongs to some fan, he simply doesnt read any fanfics or watches any fan animations. He said, that if he would ever do that and it would come out, he would never get a job in the industry ever again. So he is not risking it. If by some accident his storylines would match up with that of a fanfic, he could always truthfully say, he never read that fic and therefor never used the content of its creator. Hasbro is scared of being sued using original content. That's why everyone, who works for them, has to sign a contract, that explicitly forbids the use of content of a third party. Ted Andersson violated the contract he signed. That's why he will no longer get any jobs from IDW. Imgine the comic issue with the OC appearance becoming very popular over the years and selling idk maybe 200k copies. That DD person might just decide then, that these are alot of copies with her OC in it without her consent. She will sue IDW/Hasbro and very likely win. She will get then some amount of money per copy. Let it be 5$ which will sum up to a million. She would be dumb to not do this, if the comic becomes that popular. Hasbro doesnt want a situation like that nor does any other big IP owner. Chances are that this stunt will cost Ted Anderson his career in the comic business. Non of the big IP owners will give somebody a job, who plants legal timebombs into their works. I dont think it has that much to do with the timbomb being DD, that's just coming ontop of that. It could have been a cameo of Flufflepuff and wouldnt have been much different. What i wonder is, what will happen to the comic now that the legal timebomb is planted. I guess IDW is talking with their legal department now or with that of Hasbro what to do about it. Worst case scenario for IDW is, that the legal department will deem the potential loss of money due to a sueing by a third party higher than stopping sales of the comic in its current form and reprinting it with the panels in question fixed. Either way they will lose money or they just gamble and hope the OC owners will not sue them. ---- tl;dr ---- Legal timebomb planted in the comic, potential loss of alot of money -> violation of contract, no more new contracts.