It is their right and BYU can not refuse to have 1 of them be on ESPNU. But at the same time ESPN could chose to elevate them all to something higher than ESPNU.

Also if they chose a 2nd game for ESPNU BYU could turn it down and put it on BYUtv instead. They would not do that in my opinion as they make more money by having it on ESPNU and then they get it for unlimited broadcasts and VOD afterwards anyway.

The only way I'd ever see BYU turn down the 6th home game being on ESPNU (or other ESPN network) would be if they were trying to force a very very very bad time slot/day of the week to do so....