Apparently, the family on Sister Wives was investigated by the government, even before the show hit the airwaves. According to Perez Hilton, the government doesn't have enough time and resources to go after polygamous families, so most aren't brought to court for their actions.
Kody stands by the fact that his situation is legal, as he is only married to one wife in the eyes of the law. Also, no family has been charged with the sole crime of bigamy.
The family was aware of this risk before filming the show, so my bet is that they've got some strong lawyers on a retaining fee, and they made sure they were clean before the show began.
After all is said and done, Kody makes a great point - he states that sleeping around isn't a crime, so why is plural love? (I'm not personally opposed to it unless any participants are unwilling, for the record. Like, when he brought Robyn in, it got sticky...do the wives have an out? Are they being forced to stay? Things like that.)
What do you think?