RedemptionMan know approximately as much about what employers will or will not do as my cat. He is correct that this is not discrimination. But he is dead wrong that most employers will not verify salary. They most certainly will. Even those employers who limit themselves to name, rank and serial number will almost always verify salary. If RedemptionMan's employer does not, he is in the minority, not the majority.
Does this mean I approve of lowering an offer in this way? No, I do not. But it does not violate any laws.