“labor law already prohibits employers from requiring employees to conduct work-related activity when they are off-duty”
Maybe for some, but certainly not for most salaried jobs - never off duty.
If the employee is FLSA non-exempt, then the employer must pay or compensate any time worked outside of duty hours. Exempt employees must be compensated, not paid. And the employee can refuse to work when off-duty.
Where I work, the issue is that many employees work extra time and we are trying to minimize that activity. I get talked to also, because I don’t want to leave work until my work is done. I don’t care, I’m on a salary.