This is exact scenario where the rule is unfair and needs to be changed. The defense was getting gashed, doing nothing, the offensive player fu,blew the ball on his own and all of a sudden defense gets automatic possess on the 25??!! The defense did nothing to deserve the ball. Rule needs to be changed, in college too.
If the defense forces the fumble, then the offense should get the ball back at the point where the runner lost the ball.