The law is open to debate. That is why we have lawyers. If it wasnt we would not need so many lawyers.
If I got my arms tattoo'd and was told to cover them up I am sure I could get a lawyer who could argue my case in front of a judge and I would have a chance of winning. Maybe not in front of every judge but certainly in front of some.
My point is, you as an employer cant bend me to your will. If you try to I can bring a case against you. I may win, I may not. But if I put up a determined enough case/fight it will cost you more money (plus time and headaches) than me.
Still want to force me to cover my arms?
What's your point? I could ask you to do anything and you could bring a case if you were so inclined. What does that prove? Doesn't change the fact that employers are allowed to have a dress code. If you want to be a rebel, that is up to you.