First, Weightlifting is not purely about bulking up, it can be about keeping your body in top physical form. No one will dispute that some lifting does little to help someone throw a football, but developing your core and adding strength can help with fatigue allowing him to throw with power during the whole game. IT also great for injury prevention...for a player who is not only hit often int he backfield, but also has been asked to tote the rock from time to time and can only help. I also love how everyone assumes him holding dumbbells automatically means he is working upper body when there are numerous exercises using those that develop the core, explosive power, and the lower body.
Spending time in the weightroom with his teammates is a leader trait, showing he is wiling to put in the effort in all aspect of the game spending time sweating with his lineman, RB's, TE's Defensive players....lead by example...