Confused on legal threat

Hi!! I have a question about the whole controversy on Kody going to be potentially arrested for being polygamist. I’m just confused legally on how that could happen. He’s only legally married to one person which is Merri at this point because I’m watching season 2. They are packing to move to Vegas because of the threat of being arrested, but he’s only married to Merri legally and the rest of his marriages are spiritual, so I’m confused what they’re doing illegal. Legally you can have as many baby mamas as you want and you can live with as many women as you want.