When they rely on valuable TV spots for their high profile games, they will not want to turn down a Sunday slot because BYU is one of the teams.

And take a look at how many conference championship tournaments include Sunday play. Could they schedule around it like the WCC does? Yes they could, but if there is another no hassle option to invite into the conference other than BYU, they'll take it.

I think football-only is the easiest option.