The employer can do anything he wants. Especially if the other people do a different job than you and he thinks their job is more important for his company to succeed/make money. Or he may think/find that they work much harder than you and even possibly, complain less. You can ask him why he did it. If you are not happy with the answer you can either leave or file a complaint with the Labor Dept for unfair work practice. But you better have a good leg to stand on if you do.