It's important to know if someone neg reps you. some scammer neg repped a ton of people and I didn't even notice it for months. If they aren't in your circle you won't even see a change. I don't goto my trust that often as I'm sure most people don't. See what I mean now?
I still see no issue because as you mentioned, the trust won't become red because the guy isn't from DefaultTrust, while doing transactions with other people, they don't pay attention to those ratings unless they are from neutral or green members. If he is a scammer then Its likely that someone opened a scam accusation against him and he got negative red trust in his profile, moderators can't change anything even If that user start giving multiple neg rep to different people.