I would imagine that due to their situation, they are very careful to follow all tax laws to the letter. If how you file and what you claim is legal, then I don't have a problem with it.
I'd be willing to bet they have a good professional tax planner. There are some very lucrative legal "loopholes" for taxpayers with of a lot of dependents.
if their marriage was considered legal, then they wouldn't be filing separate.
Your dependents have nothing to do with legal marriage. Fifteen kids are fifteen tax deductions whether you are married or not. I highly doubt that Kody files a joint return with anyone but Meri, his legal wife.
Although legally married, I file separately from my husband. I get a better return.
I know a bunch of legally married folk who file separately for the better return (I did). Being married doesn't help your tax situation much. As for dependents either parent can claim them. Lots of divorced/never married parents will alternate years on which they claim the children. I would guess they sit and look at how to maximize the children or have sat down and looked at that.
I'm curious if people here have a different opinion of polygamy after watching this show. I never thought about it much before, but now I see it like same-sex marriage: why shouldn't it be legal? This family is a lot less dysfunctional than supposed traditional families, and if Kody is allowed to legally have four wives that doesn't affect the rest of us. I'm just wondering why it's so taboo.