The thing is this...in Italy, most men like to be clean shaven. It really is few and far between that you'll see Italian men with facial hair. They see it as dirty or unkempt. Same goes for tattoos. Unless you're in jail, a biker or a footballer, there's still a lot of hangups on that. So, it's not so much as it's not allowed in Cosa Nostra as it is a cultural thing. They still see facial hair as sloppy. And when you look sloppy, you act as such. I believe the same applies to Italo-American mafioso.