Posting in ALL CAPS, posting fake news or insults to other members of the community, posting to incite anger, etc. does not make for good discussion, and hurts the sense of community. That, and bringing that to other subreddits via brigading, etc.
I mean is reddit going to start deleting every sub that becomes popular because it will have too much stuff on /r/all? What kind of reasoning is that?
The real sensible thing to do would be to recognize reddit as a platform for open and free discussion. saying the wrong things or in the wrong way should be as far away from the admins concerns as possible, so long as the org itself is not threatened. I think reddit is far too obsessed with /r/thedonald, to the point where their obsession is actually becoming tangibly harmful (see: this post)
And one can argue that the trump subreddit does the same in light of Twitter allegedly not being bought out because of the hate and toxicity. If Reddit can't secure investors and funding because of notoriety, they have to do something.