I've always wondered why pants have topstitching on the inseam instead of the outseam. While sewing Henry's s/s collection I didn't find the reason, but I did find how to make jeans have the manufactured topstitching look.
1. When you are putting your jeans (or shorts) together, assemble the front and back pieces separately.
2. Sew the front and back together at the inseam and topstitch seam allowances towards the back.
3. Sew the outseams together. Topstitch seam allowances towards back only to the base of the pocket. This is exactly how manufactured jeans are topstitched!