The libertarian point is that the illegality and attendant marginalization of polygamy pushes it into isolated, authoritarian, quasi-state cult compounds where these kinds of crimes are most likely to take place.
That’s Will Wilkinson and the point reminds me of recent party debates on drug legalization. I don’t mind legalizing polygamy (though I disapprove of the practice), but would such legalization prevent an FLDS type of episode? Maybe the goals of the perpetrators are rape, abuse, and power-mad intimidation, rather than polygamy per se ("polygamy: merely a means to an end.") In that case polygamy legalization won’t limit their ability to set up isolated, authoritarian, quasi-state cult compounds for their nefarious purposes.
Alternatively, if illicit polygamy is a marketing point that draws people to the compound in the first place, legalization may well help. Oddly legalization helps most when the religious belief (in polygamy) is relatively sincere and the abuse accumulates through evolutionary processes of increasingly bestial behavior; legalization helps least when the religious belief in polygamy is for cynical reasons of control and could easily be replaced by some other marketing point.