Before standing on your rights, consider that (in the US) employment is "at will" and they'd certainly be within their rights to fire you if they suspect you of malingering. Pick your battles.
Re the comment about "wrongful termination": Nothing wrongful about it. In the US they don't need cause to let you go... but making them suspect that you are abusing the benefits does provide them with cause. You aren't required to reassure them, but they aren't required to employ you. I really recommend working with them rather than against them, unless this is important enough to you that you're willing to leave over it.
Rules in other countries may be different. Your contract may say something different. But that's the norm here.