I've "heard" that it's because they didn't want to spring for higher performance tires so needed to limit it to the tires rating.
Now, this could be speculation, but it is a ford and they have been known to cut corners (cough, cough).
A 115 limit is pretty sad. Imagine the point-by's at the track!
Track Rat #1 - VF supercharged e46 bimmer
Track Rat #2 - Kona 5.0 Premium | 3.73 MT82 | 400A | glass roof | saddle int. | Brembo Pkg | Com. & Sec. Pkg | camera | HID | Nav | no stripes | MM camberplates | Bilstein springs & Eibach dampers | Stock 368 RWHP @ 6700 / 354 TQ @ 4375 - Build Date 6-17-10
Sunny Day: Kawasaki 1600 with hypercharger and tune.