Very interesting, and I agree with the others that it seems it is a simple dress code requirement.
I have actually experienced a similar issue, though not employment related. Our local rec pool that we join for the summer has a no-clothing allowed in the pool rool - swimsuits only. My friend's son burns VERY easily, she obviously uses sunscreen liberally, but at the beach, etc. he always wears a shirt. A few summers ago (their first at the pool), the lifeguard had her remove the shirt - told her he could only wear swim-suit type tops (like those surfers wear, etc.). OK, no problem, that's the rule. They state that it is for safety reasons, clothing can get caught on things, etc. All good. Not 2 days later, I'm at the pool and there are 2 women, I'm assuming Muslim, IN the water in FULL cover, only their eyes, hands and feet were visible, everything else was covered in black cloth. I was stunned, specifically because the lifeguards indicated the rule was for SAFETY reasons - how can an exemption be made then??? (Never mind the fact that I can't imagine how heavy that must have been for those ladies - yikes!!) I told my friend and let her decide on whether she wanted to follow up with aquatics staff. She bought a swim shirt and chose not to say anything. It was very interesting to me though, I'm sure the rec was worried about keeping everyone happy, not discriminating, etc., but it seems that if you use safety as the reason....