100% completely normal.. don't stop him, he is cleaning it off. you don't want hair rings..
as for not noticing all of them doing it, i have 2 males, and i catch chin chin doing it , but never bo.. i just figure he does it when i'm not around or stops really quick when he hears me coming. chin doesn't care.. typical male.. haha
as for hair rings, you should be checking for them. i forget how often they say to do it, but hair rings can be very dangerous if not caught.