That's one reason why a no bot policy wouldn't work.
A bot posting articles from your favourite news site is actually useful. A bot which prints out Trump's tweets, burns them and then tweets out the video [@burnedyourtweet] is mildly amusing (and some pretty smart engineering). A bot which automatically replies to every tweet by $personality with a meme is usually an 'orrible troll, but occasionally amusing and often owned by someone who's very convinced the First Amendment applies to Twitter. A retweet bot which does a plausible impression of an angry Trumper might be indistinguishable from the real thing without investigating its network, and I don't think eliminating the real thing would be a great move for Twitter to make.
The other reason is that Twitter's review process is more or less completely random in its responses (I'm reminded of the girl that reported literally the same abusive tweet directed at her from three troll accounts, and received a different response for each one)