They can, though the employees would be able to claim unemployment if the job was remote and then changed to on-site but if the job was on-site with a temporary remote policy the employee wouldn't have a leg to stand on there and could be dismissed for cause.

In the US, what you can and cannot fire someone for is complicated and counter intuitive.

A low performer that is part of a protected class is hard to fire because you need to have copious documentation that they were dismissed due to poor performance and were not targeted for their protected class status. This is a good thing and prevents unscrupulous bosses from firing a woman for getting pregnant, targeting people of a particular race, religion, or gender, or any number of other awful things. Those things will only come up if the former employee sues, and many will not, so some bad bosses or companies get away with this while others end up in court because someone that needed to be fired is crying discrimination.

On the flip side, if it falls outside of those protected classes, you can fire someone for any other reason or no reason at all. "I woke up in a bad mood and picked a name out of a hat to fire" is legal. You may get a fight if the person you picked claims discrimination on one of the protected classes and you have to explain to a judge that you're actually just a bad human and not discriminating, but it's allowed.