I just started watching this show recently, with the Season 1 DVDs. I haven't watched any of season 2 yet, but I figured this was a good place to ask.
I understand that the polygamy as portrayed in this show would be taboo, and could be a PR nightmare for Bill and his store if it were to get out, but how would it violate the law at all? He's only legally married to Barb. What he does with the other women, as long as he tries to claim none of the legal benefits of marriage to them, would seem to be irrelevant.
The states each have a mish-mash of laws relating to marriage, common-law marriage, cohabitation, adultery, bigamy, and even unmarried sex. Many of these simply aren't enforced anymore, and probably wouldn't last long under Supreme Court scrutiny if they were. Bigamy and adultery are only crimes for married people, so avoid getting married even to the "first wife" and that'll never apply.
Generally speaking, if you avoid getting a marriage license, don't represent yourselves in a way that you might be considered a common-law marriage, don't represent yourself on official documents as married and avoid labeling your relationship with the term "marriage," it's pretty unlikely there's anything anyone can do to stop it even if they were so inclined to try.
Generally it seems it's the age of consent laws that get people all riled up against polygamy these days... the guys who take child "wives" and everything related to that concept of "brainwashing" the children to accept such things for themselves. There's also the overabundance of males in such communities that's often dealt with by tossing the male children out on the streets -- too few "wives" to go around, and various child endangerment laws that would be enforced to prosecute such practices.