Spammers/scammers use Tor. And they use it in a big way.
One of the issues with total Tor anonymity is that yes, the "bad guys" will almost certainly come. Because they can.
I am not saying that banning Tor outright is the best thing sites like HN would do, but seeing the ratio of regular Tor trafic vs. spammy Tor trafic on much smaller site myself, I can't blame them; almost anything that comes from Tor is spam.
Sad reality.