I think it is a false equivalency and two completely different scenarios. Lebron left Cleveland in one of the most arrogant, outlandish, and childish ways a professional athlete can leave a sports team. It really wouldn't have been even half the spectacle if he didn't act the way he did, Miami wasn't competing for championships when he left Cleveland, and the roster turn over was extreme from 2009-2010 for the Miami heat. He then returns to Cleveland and wins the cities first championship in 50+ years, pretty much redeeming himself with Cavs fans and completing his scripted redemption story. Lebron was more scrutinized for the way he left, rather than the end result.
Durant leaving to the Golden State Warriors, is seen as one of the weakest/cowardly free agency moves in professional sports by a lot of people. He left a team that could compete for championships, for a guaranteed spot in the finals with one of his main opponents. Durant moving to Golden State greatly effected the parity of the west. What both players did was scummy to the fans of their home cities, Lebron for acting like an idiot, and Durant for choosing a ring over being competitive.
Both deserve the character assassinations they got/are getting for their actions.