This.
As for Barnett - he is a Jr. He hasn't yet said he is leaving for the NFL (... though he probably is ...), thus him making a decision not to play just in case would not be appropriate.
Some players will look at the game as another chance to showcases their skills.
I believe also that not playing sends a clear message that they are afraid of getting hurt. If they feel this way about the bowl, what is next - the last game of the season doesn't matter, so skip it ... as soon as it is apparent their team is not going to a bowl or will not be conference champs - check out?
And then there is the playoffs, how long before someone gets hurt there and the players say - the NFL is greater than a college football championship with their teammates? So they decide not to play in those as well.
These players should play. The universities have given them the opportunity to showcase their ability for the NFL and get a degree at the same time. They have contributed to the success of the team.
This is the equivalent of someone leaving for another job, giving a two weeks notice, but refusing to transition to those who must perform their job after the leave. It almost always comes back to haunt them at some point.