There are cases where deserved or not, negative trust has been left for a user. Then another user comes along and deems that negative trust as wrong/unfair/inappropriate or as a breach of the trust system and decides to leave positive trust for the same recipient stating it is to counter previous feedback.
I think it is an inappropriate use of the trust system for anybody to give positive trust ratings to users just because they received negative trust that was (or could debatably be deemed as being) in breach of the trust system.
I disagree with this premise.
If there is no fact dispute, but Sally believes Bob is "high risk", Sally will leave a negative rating against Bob. Sally gives a lot of "good" ratings, and the rating does not appear to be in bad faith, so it would not make much sense to exclude Sally from the DT network. However there is still an "injustice" against Bob. Sam wants to correct this injustice and asks Sally to remove the negative rating, but Sally refuses. The only option is for Sam to leave a positive rating on Bob's trust profile with a note saying he does not agree with Sally's rating.
Anyone doing due diligence prior to trading with Bob will see both ratings, and can act accordingly. If Sally subsequently removes her negative rating, but for some reason Sam does not remove his counter rating, someone conducting due diligence would ignore Sam's rating.
This is an excellent scenario but what happens if Sally decides to take further action against Bob to counter what Sam wrote and then took action against Sam too by leaving negative trust and adding Sam to her distrust list? In your scenario it is quite probable that Sam will take action against Sally by reciprocating the negative trust and the whole cycle could theoretically continue with more DT members getting involved...
Now seems more apt to conclude what The Pharmacist said about it being pointless to debate issues like this, is spot on.