I tend to agree with Mourinho. Rescinding Gabriel"s red card is setting a precedent. He retaliated and flicked out at Costa. A stupid decision by the FA.
I am of two minds about this...
Provocation deliberately designed to lead a player to retaliate and get a red card is a real problem. A dedicated troll who is smart and canny would be able to seriously impact the play of the opposing team and help his team win. A team could even end up having a designated "troll" player who's job it is to create problems and try to get an opposing player red-carded.
That is something the FA has to tackle aggressively. So suspension of the instigator and rescission of the red card is about the best they can do - they can't give Arsenal the 3 points

But I also see your point - retaliation is bad full stop. Don't do it - no matter what. Perhaps the FA thought that Arsenal's loss was the real punishment and thus the point was made?
I don't know. An odd decision all around.