Its been awhile since I read this but I remember reading it somewhere. But that is indeed the reason.
Because Madden is NFL exclusive and the NFL has several TV partners, it would be frowned on by the others to have either as the one network used by the only NFL video game.
We have to remember they have deals with Fox, CBS, NBC, ESPN as well as NFL Network. So its more complicated than just having dual networks as DaveyJ suggested.