First, you are implying that only Mormons are or can/will be BYU fans. That is a fallacy. It is like claiming only people living in Idaho are fans of Boise State. Of course the term "fan" needs to be defined here. When it comes to TV I think it just means people who are interested in watching the team play on TV and the more exciting the team is to watch the more eyeballs the team will draw nationwide (see the Jimmer factor in recent months).

Second, bowl game TV numbers are affected by a lot of things: Time and date of a game and opponent being two important factors. Seems like one could compare full season TV numbers between BYU and Utah in a fair way (making it apples to apples for things like network the game appeared on) if one took the time to do so.