[harabat]

There was a report posted on HN months ago showing the proportion of fraudulent vs legitimate activities on Tor vs non-Tor across a pool of websites. Tor users were (slightly, IIRC) more likely to be malicious, but they represented a drop in the bucket against all malicious users. Maybe some sites that do block Tor might be seeing different proportions, I'd wager most of them do it because Tor == bad for most people.

I'll add the source when I find it.