I side with the players, Ivey and Sun, not the casino.
The issue is whether edge sorting is a legitimate technique or is it cheating.
The rules are established before the game is played. If a player uses his skills, vision or intellect, after that point, it is legitimate. When the casino bet that its cards were impervious to edge sorting (a known technique), the casino lost. It lost because it was arrogant in believing in its own design.
Besides, Sun making the request to rotate the sixes through nines should have been enough to clue-in the casino. But it agreed to the request.