The employer doesn't decide unilaterally. The employer's only free to direct the employee within the limits of the employee's contract or collective agreement (as the case may be).
As someone who claims to have managed staff, this is really something you ought to have already known.
In order to compel staff to attend or go without pay, Shell argued that the rally was a "training session" and that Trump was a "speaker" in this training. This is transparent bull****, of course.
As someone who claims to have managed staff, I can tell you that most employeeś in the US have neither a contract or collective bargaining agreement.
Choosing an offered position is based upon the position being defined by an approved job description.
This is an outline, not an exhaustive list of every possible task the employee may be asked to do.
Policies related to employees, and their job, change, and the employee is expected follow them.
Reorganizations occur, and these can impact a specific task and how it is done.
I know this because I claim to have managed staff.
To this specific event. A private employer has the right to determine the days and hours the employees will work.
No one was compelled to attend this event. Those who chose to attend were paid their regular pay for activities during their normal work day.
Those who chose not to attend could go home.
Right, wrong, whatever, perfectly legal,