I'm a Durant fan, so this is thru beer goggles, but here are my thoughts.
1st. Is there something more to this. A family issue. Ill family member. Something. This just doesn't align with his character.
2nd. Does he see next year as a do or die year and wants to utilize the entire offseason preparing and working with the OKC staff.
3rd. Lets not overstate this in reference to him, is Lebron on the team? Or several other top players?
No, which is why this is such a big deal. The best player on the team may in fact be Derrick Rose. I think with a gun to my head I'd say Anthony Davis right now especially because of his versatility, but there is nobody else I'd rather have on the team than Rose right now. He's a vet (one of two on the team from the 2010 Champ team) and looks to be close to pre-injury form. Lebron opted out. Love backed out at the last minute. Griffin did too. Aldridge backed out. And with KD out, Team USA is at a significant disadvantage vs the other top contender Spain.
As to 1 and 2, some are mentioning his upcoming mega deal with UA for $300+ mil, but I think 2 is just as important. There would be a lot of pressure on him to succeed in this tournament and if he shows any fatigue from the World Cup in the regular season, he'd be criticized. Everyone knows how big of a deal this next season is for the Thunder. It could be Scott Brooks' last year or could be the tipping point to make KD leave. It's certainly possible.
I still hold out hope for Team USA, but they are going to be at a significant disadvantage vs Spain now in the frontcourt. The only hope is that the superior shooting and playmaking ability of Team USA can more than make up the difference.