Even if it’s true, I don’t see the problem. Sure, playing in higher difficulties get you better rewards. That is the whole point of their existence in the first place.
You have to remember that the same reward means less for higher level players. The same single gold key would make a starting player very happy, but feel like nothing to a high level player. If we all get the same rewards, the high level players will feel at some point as if their advance is almost completely halted. That is why they can get better rewards through expensive armor, playing higher difficulties, guild task rewards (in top level guilds), and more gold in PVP (since they battle other high level players).