I see your point; however, since an exempt employee has no legal expectation of any additional compensation regardless of what hours they work or on what days of the week, an employee who says No to an employer's request can be setting themselves up for termination. So, for that matter, could a non-exempt employee, except that since the employer cannot require a non-exempt to work without compensation, firing the employee for refusing could be seen as a wrongful term. Not so the exempt employee. Even if what the employer asked for is non-work-related, at least in an at-will state.
In any case, our discussion can certainly provide the OP with an expanded answer to his question!