#3 The Warriors had narrowly eliminated the Thunder
It's supposedly not about how hard you're hit, it's about how hard you can get hit and keep moving forward. In this case, it's about how quickly you can change sides and go to a team which beat you.
It's bad enough going to a heated rival for whom you ought to harbor some amount of animosity. But it would still be understandable if the rivals were markedly better. There's nothing all that wrong with trying to level the playing field, even though it's not honorable.
What makes it worse is that the Thunder were not vastly inferior to the Warriors. One could argue that the gap between the two teams was infinitesimal, seeing as the Thunder pushed the Warriors to 7 games in the Western Conference Finals.
It is bad enough jumping ship to a team which beat you. When you abscond in spite of having a great team yourself, that opens the door to ridicule.
LeBron had jumped ship to a nondescript team which gained prominence only because of his and Chris Bosh's arrival.
