I understand why they went hard after Kela, and I completely agree that intentionally throwing at a guy needs to be stamped out from the game. I guess what is different in this situation is that after Kela went after Dietrich, if Dietrich had charged the mound and went after him and they had the brawl, it still wouldn't be right or okay, but it is an accepted part of the game that when a dude throws at you, it is time to rumble.
What Garrett did, was take a situation that had somewhat been de-escalated from the Kela vs Dietrich incident, and irrationally decided to incite a huge brawl by running into the opposing dugout. That is a different layer, and I don't think with that kind of action you can hide behind the fact that Kela started it. Yes Kela started it by throwing at Dietrich, but Kela didn't really start this giant brawl, that was all Garrett. I guess you could make the argument that what Garrett did was actually safer than if he had tried to bean the next Pittsburgh batter in the head.
I think whatever MLB uncovered through this investigation, it was clear to them that this was an issue involving Pittsburgh first and Cincinnati second, despite the actions of Garrett, Puig and Bell and that they needed to be punished the hardest. I just think that Garrett should have gotten the biggest suspension.