I don't think anyone advocating the 3 months thinks you need to keep them doing something. You just need to keep providing the benefits (salary, health if in the US, visa, etc.) for 3 months. Even in the EU you don't have to let someone come in to the office during that time.

I suppose if you had really, really, really good meals provided, it would be reasonable to argue about whether that was a benefit or there for people in the office.