I see employers do this all the time- when they want to save money they freeze or take away stuff from the non-unionized employees while the unionized ones keep on getting their raises.
I realize the contracts say they have to keep getting raises but this was a sensitive issue where I work. During the last union negotiation our management proposed a lot of money saving measures to the unions that got beaten back but were imposed on us non affiliates. Those spineless management bastards only pick on those they can.