The sad truth is that for large classes of Internet abuse, IP is a pretty strong indicator of reputation, positive or negative, and the reputation of an unknown (which includes exit nodes of VPNs and Tor nodes) tend to be well below average.

Any _content_ based semantics are far more expensive than a DNS query (or better: a local IP reputation table lookup). It's the online equivalent of individual stereotyping based on obvious surface characteristics -- even if only unevenly true, they're a cheap heuristic, and, as the saying goes, the thing about stereotypes is that they often have an element of truth to them.

It's not just the big players who do this either. Lauren Weinstein, long-time Internet privacy advocate, has taken to blocking entire TLDs which are characterized by near total spam -- sorting out who's legit is just too much work. And of course DULs are heavily used in email configurations -- very few sites will accept email from such spaces.

I really like the idea of Tor and really wish it could be tractable, but absent some alternate means of sorting out the good guys (and keeping bad actors off of Tor, without penalizing others, and remaining consistent with goals of privacy and anonymity), that's going to be difficult.