The thing is, you're assuming those in polygamist marriages are 1. consenting, and 2. adults. They frequently are neither, and before you say "then that's illegal" understand that the Lakota and Suix Nations are actual sovereign nations within the US with their own set of laws, courts and law-enforcement structure. State LEOs can only enter tribal land under very limited conditions and routine enforcement of civil law isn't it. Short of a Federal warrant, a State LEO has to be actively cashing a murderer or similar, and even then still has to stop the pursuit when ordered by the tribal police. Sanctioning polygamy typically results in families selling young daughters in pre-arranged marriages.
Opposition to polygamy has nothing to do with consenting adults.