Do you have experience with employment law?
An employee could argue discrimination, but they'd have to have evidence that it was due to a protected class to have any success, and those cases are notoriously hard to prove. In every state that I'm aware of, they can fire people selectively for not coming into the office, while keeping others employed.
This is true for contract workers, but I believe we're talking about W2 employees, who rarely have a contract if they're not part of a union.