I'd say it depends a lot on the legal framework of employment contracts. I don't know how it is in the US, but in Germany you sign a contract with your employer stating „I will work X hours a week for you“. It's not about the output.
If you work less than X hours, you commit fraud and breach the contract. Your employer can then fire you for an „important cause” and typical legal protections don't apply. If your employer forces you to work more than X hours without compensating you appropriately, he breaches the contract and you can sue him for that.
So obviously, there have been a lot of legal cases like „Does changing your clothes count as working hours“, „Does driving to a client count as working hours“, „How do working hours need to be tracked“,...