To call it 'dangerous' is a bit of a stretch..maybe annoying. The easiest detection system is that the tweets of an account with a lot of fake followers will have very little, if any, interaction . I'm kinda irked stuff like this makes to arxiv but you still need the 'two referrals' system to upload stuff. I can't image the worst math paper is much worse than this.
I agree that the easiest detection system is to look at engagement of tweets...I have a few thousand followers and I average about 2 to 5 favorites/retweets...so if I see someone with 50,000 followers and less than that average, I get a little suspicious. But most people don't think to look at this...and news outlets are quick to jump on what seems like a valid tweet from a valid account. The reason why we don't see much of this happening is because most of the people who buy followers probably aren't hoping to do anything particularly malicious or disruptive.