Your problem is with the doctor, not your HR. They have no legal obligation to EVER provide light duty. It may or may not be foolish for them to refuse it, but they may legally refuse to do so.
I can completely understand why, if your doctor put you on light duty, they would want the same doctor to certify that you are able to return to full duty, since if you were to reinjure yourself they could be held liable. It wouldn't be the first time someone went doctor-shopping to get the response they wanted and later re-injured themselves. There's no reason your doctor can't sign at the bottom of the page or in the margin.

