very easy:
No for tattoo = Yes for beard
One thing is the same in their reasoning: someone, somewhere might look down upon people with tattoos or people with beards, so no JW is allowed to have either.
They don't seem to worry about people who look down upon religions that put forth pharisiacal rules, though, and my guess is that those people outnumber the few that would reject the ultimate religious truth just because it was brought to them by a bearded man.