Unless you have an employment contract (or union agreement) that guarantees your job duties, then you have no claim here. The fact is that a business can set the conditions of the workplace much as it sees fit (absent some form of legally actionable discrimination). Accordingly, your employer can add extra job duties and for no additional compensation. If you refuse to take on this added responsibility, you can be terminated. In fact, you can be fired for any reason or no reason at all, with or without notice.