I'm going to try to explain the difference as well as I am able, but it may fall on deaf ears.

Ultimately, Sunday Play as an issue could just be a matter of perspective. It's an issue for BYU because it is rooted in your faith and ideology. It is a non-negotiable because of religious reasons and is therefore a religious issue. If BYU was left out of expansion because of Sunday Play, BYU fans would argue that it was necessarily because of religious reasons.

For the Big 12 perspective, they could not care in the slightest about WHY BYU doesn't play on Sunday. They could be perfectly fine with BYU observing and practicing it's faith based traditions and commandments as they see fit, and not blame BYU for not being able to play on Sundays. However, the Big 12 may have a need that all of its members be available for Sunday games. Maybe it is due to scheduling, tv rights, tournaments, whatever. BYU Sunday Play status then becomes an issue, not because the Bug 12 has a problem with Mormonism in particular or in BYU being a faith based school, but just because the Big 12 has to have teams play on Sunday.

It's two sides of the same coin, but I feel the intentions make a difference. If BYU were rejected, I don't think it will be because the conference is filled with religious bigots who hate Mormonism or something. I think it will have to do with the fact that the two parties just have irreconcilable goals.

At the end of the day, the result would still be the same. I hope Sunday Play isn't an issue for the conference, and greatly respect BYU for sticking to their morals. (I wish Baylor would have the same dedication and commitment.) It doesn't serve as much comfort for BYU fans, but I hope the difference in perspective is enough to make it clear this isn't a bigotry or depraved heart issue, it's a pragmatic issue.