Why can't I leave negative feedback for a buyer?

Since May 2008 it is no longer possible for a seller to leave a negative or a neutral feedback for a buyer.

You are also not allowed to give a 'false positive', IE a positive rating with a negative comment, as this is removable. The best thing is to leave no feedback at all, otherwise you're just increasing the buyer's score, even if the transaction was negative from your point of view.