I'm gonna take a stab at this and say:
In Japan tattoos are heavily linked with delinquency and the Yakuza, and although a lot of younger Japanese don't actually mind tattoos, it would create a bad image in Ayumi showed her tattoos in official photo-shoots, well, except for Secret, but that's an exception.
Now I know that Namie and Mika show of their tattoos a lot, but I know that Namie seems to have that bad girl image imo, whereas Ayumi doesn't have a bad girl image.
That's what I think anyway.
