Thanks for drawing attention to this. From the comments on WOT, it seems that Nullrefer is being reported on adversely because it is effective, hides referral headings and so can be used by phishing sites to hide their real origins. The topmost comment seems more realistic:
"This site effectively hides your IP & browser referral-header, & can be abused like any other service, e.g. GeoCitys, TinyURL, HideMyAss etc. Please comment on the target site, not the middle-man.
The service is simple, effective, free & not criminal, so please review your rating, if you gave a poor one."
Apparently, this site also has a poor record for "child safety". Yeah, kids can use it from their school PCs without getting caught. Also "vendor reliability" which is weird for something that is free.