People make too much of players changing teams. That in itself says nothing of their character. Jealousy is what leads to so many people getting upset with this.
On the other hand, there are sometimes missteps by players that make it worse, like The Decision or Durant's comment a few years ago about LAL and MIA.
Players benefit from image. It helps them sell sneakers. Since they all try to get big sneaker contracts, I think it is fair game when the press and fans generate these narratives. Intellectually, I understand that Durant went to a good working situation, just as the rest of us often choose to do. But when fans discuss his greatness, people will remember this move for a while.
Thing is, if he wins a finals MVP and a few rings in GS, he will eventually just be remembered for his gaudy stats and his rings. Then again, both he and Curry have hurt their changes for league MVPs going forward since both of their scoring averages will drop. We all saw how that changed perceptions of Bosh and Love.