What should Twitter do in this case? You say "their damn job", but their job is to apply their content policies, which they say they are doing. Given only the tweets linked in the article, I can see how a moderator would look at them and not find that they are obviously breaking any rules. They may be false accusations, but Twitter has not agreed to do independent investigations of every report by a user, or take on the responsibility of trying to guess hidden motives of the accused. Twitter moderation is not a sanctuary, or a refuge, or a crusader for justice, it's a pro forma box-checking policy and never claimed to be more than that.