I don't think people are necessarily against preventive feedback as you call it, but just feel like tagging people for a scam that hasn't happened yet is an incorrect use of the trust system. Been saying for a while that we need 2 different trust systems, 1 for reputation and 1 for trading, or we just need to adapt the current feedback system and let it be used for more than trades.
If there were multiple trust systems it would explain a lot but to implement it is one thing but to simplify it if it were implemented would be another. This would add more work for the admins and I doubt they will contemplate adding this as a feature.
There are times when negative feedback before a scam is invaluable.
This is true but there are problems to face if/when other members do not accept a scam was in the workings therefore would query and question the feedback. If scenarios such as those were to be played out it would empower scammers and would-be scammers knowing they could get DT and non-DT members to fight and argue between themselves. If that were to happen the eventual winners would be the scammers not the community trying to protect this forum.
Personally I think DT members need to be aggressive on this forum; if they weren't, all we'd have is an enormous lineup of scam victims and other assorted fires that needed to be put out. You know that old saying "An ounce of prevention is worth a pound of cure"? It's absolutely relevant to bitcointalk and scam busting/prevention.
As matters currently stand, I definitely agree with you however, sometimes that aggressive approach creates more problems when sufficient back up is not forthcoming. Rather than run the risk of getting bogged down in the quagmire of forum politics, some members would prefer to take a much more lenient approach.