I don't think it comes down to what is socially acceptable and what isn't, it's about two cars of the same model/age/condition/miles being compared back to back price wise with one being not on the hit list and one being on it. The one on the hit list WILL be harder to sell and so is cheaper to entice it over an unregistered car.

Daft thing is that both could have been bumped in their time but one was written off and one not, but the recorded tag means it is less desirable and you know for sure and you can never be 100% about what was done to sort it and how well unless you saw it being done.