As said before ratings are given based on the behavior of the user who's suspected of scamming and the experience of the user giving the feedback. Those on the receiving end can defend themselves. Can you provide an example where
1) someone was suspected of scamming
2) they provided information to show they were legit
3) the negative feedback did not get removed and
4) the person that left the negative feedback is (still) in the Default Trust list?
To be completely honest, I cannot provide you with exactly the scenario you request. But this comes in part from the subjectivity of many of these qualities.
1) "Scamming": one person's "scam" is another person's business. Vod considers anyone selling MSCorp stuff to be scamming, but many people consider that an honest day's work (I'm personally ambivalent on this one, I just use it as an example of the interpretive nature of "scamming").
2) "providing evidence": why should one have to prove one's innocence in the face of subjective accusations. For example, upthread ACCTSeller decided that since I'm suggesting that the trust system be "weakened" I must be an alt of a "scammer" or in some way be up to no good and he therefore neg-reps me (btw, no problem, he's not on default trust, no one will see his trolling feedback). If he were on default trust, why would I have to "prove" or "provide evidence" that my criticisms of the trust system aren't "a scam". And how could I ever do so, given the subjective nature of "scam".
So, I don't have (3) or (4) since, as I said above, I don't have a perfect example to feed you.
In general, in this thread, I see people saying "yes, the default-trusters are a kinda unregulated police" and others saying "nah, they are helpful". I think that I want to acknowledge some truth in both sides, yet I still think the forum could be a better, more drama-free place if we could do something to reign in things a bit. As I said earlier, even changing the text from a big red "WARNING...CAUTION" to a yellow "This person has received negative feedback from someone on your trust list" would be a big improvement, I think. Especially because using the phrase "your trust list" would hopefully invite people to ask themselves "my trust list? I have a trust list?" and then go ahead and figure out how to use the trust list properly for themselves, if default trust is going to continue to be an opt-out system.