Its no more illegal than that same employee using the rest room.
The employer (or his or her agent) can assign any employee duties that are required to be done to operate the business as the employer desires.
The other side of that coin is, an employee isn't a slave.
Should the employee become dissatisfied working for any employer, he or she can simply resign or quit.
Illegal as in a criminal offense? No. A civil wrong such as a breach of your employment contract if you have one or a demand that is outside the scope of your employment agreement - maybe.