Sure, but the criteria for leaving negative trust are (as you know)
Not: "I believe this person doesn't conform to my code of ethics".
There aren't very many proven scams running signature campaigns.
Someone running a sig from a site which others consider likely to be a scam may reasonably say "Until it's proved that they are, why shouldn't I take the money?"
Does that warrant red numbers and a label saying the above?
There is no defined reason to leave a negative trust and DT members make their own decision while leaving a trust rating. If they find a user to be untrustworthy (for any X, Y, Z reason), they'll leave a negative for him. I've seen people giving ratings for the signature also they promote as it's a scam website. Similarly people promoting ponzi webistes (not owned by them) are also marked as scammers. It can be termed as a trust abuse but that's the way it works and we can't help it even though we disagree or agree with it.