I think you have a legitimate question. At what point during the season did the games stop mattering for McCaffrey? When he realized he wouldn't win the Heisman? When he wouldn't be a serious candidate for any POY awards? When WSU blew the doors off Stanford? When they realized they couldn't play in the pac 12 championship?
Of course it is a legitimate question.
But also from the perspective of the team and the program, what is the difference between regular season games and a bowl (once a team is eliminated from championship contention)?
Mostly games for teams not in championship contention are played either to achieve bowl eligibility or to position for a better bowl. You can't say that games played using the motivation of achieving a bowl are less important than the bowl itself. That is nonsense.
At least since the 1980s...
Bowls count the same as a regular season game to a team's overall record.
Bowl statistics count towards team and individual totals.
Bowl results influence rankings
Bowls afford extra weeks of practice
Bowls afford extra weeks of media coverage for the program
Bowls typically receive more media attention and are broadcast to wider audience than regular season games
Bowls come with perks for players.
Bowls are never declined or skipped unless as part of a punishment.
Bowls wins come with a trophy and often rings.
My question isn't whether or not a player has the right to skip them, but if they do, why is it any different than skipping the last few regular season games of a team not in contention? The class/graduation argument is a little disingenuous since many players, especially those that might be prone to do this anyway, are more likely to leave early or don't really attend college to earn a bachelors degree in the first place. It is a slippery slope.