Touchback or 1st down?

[BillF]

One other thing I noticed about this play... it looked on replay like the ball was fumbled at around the 1-yard line, and went forward into the end zone and out of bounds. Since the facemask penalty would be enforced at the end of the related run (the spot of the fumble), it seems like the penalty should be half the distance from the 1, which would be the half-yard line, and the Bears should have 1st-and-goal from that spot. But the officials enforced the penalty from the previous spot (the Carolina 7), giving the Bears 1st-and-goal at the 3 1/2. I'm curious as to why the penalty was enforced from the previous spot and not the spot of the fumble (end of the run).