There's underblocking and overblocking. Underblocking is allowing TOR traffic through, but also letting TOR traffic flood your servers.
It's obvious that if you have a flood of nefarious traffic like this then you should throttle the TOR traffic. Overblocking is outright blocking TOR with no reason other than because you can, and it leaves many legitimate users frustrated and feeling like the site just self-censored itself.
It would be suitable in these cases to strike a happy medium and allow some TOR traffic through, but throttle suspicious-looking requests like mini 'swarms' of TOR exit IPs hitting the site all at once, which I think HN does, because some TOR idens work, whilst others do not.