Just about the strangest argument I have heard in defense of this practice came from a player who criticized my games for being too brutal. He said I have a screen for a reason: keeping the rolls secret gives me the discretion to change them if I like, and not doing so is unfair (somehow) to the players. I replied that if players caught wind of me doing that, all credibility goes out the window; and if I save one person's character by lying about the numbers, the next time someone else is in peril (and every time after that) I am stuck in a rut doing the same thing or, worse than having my integrity question, I will have the unfortunate reputation of playing favorites.
It was a weird conversation. He was just butthurt that I TPKed them with an avalanche.