For SOS, adding a non-P5 team is bad because the expectation is that such a team would lose all their games playing against all the B12 teams.  However, if BYU is better for the other B12 teams for their SOS, that necessarily means that BYU wins quite a few games against B12 teams.  That works out for the top B12 teams, but only if they don't lose to BYU.  If an upset occurs, the B12 loses in terms of any playoff consideration.