Why does one need to place counter-feedback with positive trust thus be in breach of the trust system when their premise for doing was to point out that someone else who had left negative feedback had breached the trust system?
If users feels so strongly about upholding the trust system there is not justification for them to breach it even if other users have breached the trust system where negative trust has been given. Why would someone then breach the trust system themselves just to point out another user has done it? Yes I agree two wrongs do not make a right. Just because one user sees negative trust as unfair it does not mean positive counter-feedback is the correct response when a neutral could suffice.
Going further, why is leaving negative trust deemed more of an issue than leaving counter-feedback? That is how it seems.
The following is based on your premise (counter to "negative trust that was [...] in breach of the trust system").
Two wrongs don't make a right and this goes both ways. One shouldn't leave positive feedback unless they genuinely believe the recipient is unlikely to scam anyone, regardless of any prior ratings etc. However it's disingenuous to paint it with the same brush as negative trust abuse. Don't neg-trust trolls so that people don't need to try to clean up your mess. Don't justify neg trust abuse with a false equivalence.
But if one does need to counter - I think a neutral is more appropriate in some cases. It's just as visible but doesn't imply "unlikely to scam". In the old pre-flag system counter feedbacks were allowed and encouraged in some circumstances but it's a bit more complicated in the new system where there is no longer a trust score that would reset with a counter rating.
Given that the vast majority of DT1 doesn't give a shit about what's written on the trust page ("high risk" and "unlikely to scam" specifically) and wouldn't use the proper solution to the problem (~), it's no great shock that some users may still resort to the counter ratings. That doesn't make it a "breach". It makes it an imperfect last resort workaround for what should have been avoided or prevented to begin with.