It wasn't actually. The game is now filled with fishy production twists that are geared to helping whomever they want to see win.
That twist was built into this season before it started, and was revealed before production knew Ben would absolutely need it. He was one upside down U away from winning the final immunity challenge, and then it would have probably come down to fire between Devon and Chrissy.
I actually love the twist, but I do agree the castaways should have known about it from the very beginning. It sounds like the twist is back for Ghost Island, and hopefully those contestants are made aware of it at the start of the game.
Going to fire in the finale has always made for great TV, even when the two contestants can't make fire (Becky & Sundra/Rodney & Carolyn). And so many "robbed" fourth (or third in old school seasons) place "best to never win" finishers like Rob C., Jonny Fairplay, Malcom, David, etc. would have benefitted from this twist getting a chance to argue why they deserve to win.
Either way, I don't think it's franchise-ruining like so many fans online are saying, and Ben is a great winner regardless of the way this twist benefitted him. After Ghost Island, based on the fan response, I think this twist will go the way of Redemption Island and not return. If it does stay, which is fine with me, it just needs to be known about by the contestants so they can plan accordingly.