Once they made the rule that a fumble that goes out of bounds forward is placed at the spot of the fumble and is retained by the offense, that should also hold true here. That rule changed because of the famous Raiders fumble forward play. So that whole touchback rule was in place to discourage the offense from intentionally fumbling into the endzone when it was legal. The problem was if it was 4th down, it was worth the risk and that brought on the spot of the fumble rule. Now if you fumble backwards, the ball is placed there but you keep possession. This even happens if you fumble backwards into your endzone, the ball is actually retained by the offense where it went out, but it is a safety because that is what happens when an offensive play ends in your own endzone.