Sure, the NFL can flex games early in the season, but who wouldn't want to watch Peyton play in his first game for a new team?
Duante Culpepper suffered a similar fate in Minnesota when he was still considered a good QB, and his first game with the fish was on primetime in week 1.
Also, the Pats were placed on MNF during week 1 of 2009 because I'm sure a national audience wanted to witness Brady's return from injury.
With Peyton Manning you get both (coming off injury/playing with new team). Who wouldn't want to watch his first game back?