Can a general manger make you shave all facial hair even though the employee handbook says it's ok?

When I interviewed for my current job I interviewed with a trimmed beard. I just happened to ask my manager what the policy was on facial hair. Her replies was simply that facial hair was the GM's pet peeve and that it would be better if I did. So, I decided to read through the employee handbook that my manager gave me. It clearly states that facial hair is allowed as long as it's neatly trimmed and as long as your facial hair grows fully (meaning no patches) and it had to be grown during a vacation. Now, my question is this. Wouldn't it be illegal for a general manager to ask all his male employees to be clean shaven because he simply doesn't like facial hair when the employee handbook says its allowed? Isn't that considered border line discrimination? Specially when all the male employees had some sort of facial hair during the hiring and interview process? Now they threatening with writing people up if your not clean shaven even after you followed the guidelines to grow facial hair?