Many other programs' finals end a week sooner than BYU, including Saban's Alabama and UW. That in combination with the fact that many other programs, or at least the ones everybody compares BYU to, have bowl ties to post-Christmas Day bowls would seem to make it significantly easier for them to get all these "extra" practices in.
Academic eligibility is every bit as important as practice, technically more so. And since the Fall semester is likely most football players toughest due to being in season, doing well on the finals is likely their last chance to secure the grades they need to remain eligible.