Polygamy isn't just against the law in Utah and Arizona. It's prohibited by the State Constitutions of both Utah (Article Three, Section 1") and Arizona (Article 2, Section 20. Why? Because of the wide-spread illegal polygamy by members of the Church of Jesus Christ of Latter-Day Saints at this time Utah and Arizona became states. In fact, in the case of Utah, the LDS Church had to give up polygamy for Utah to become a state (we won't get into the deceptive 1890 Manifesto, and the following Second Manifesto of 1904).
The anti-polygamy provision of the Utah Constitution may be unique among all provision of any kind among all the fifty states. It specifically states that the people of Utah can't change that provision of their own constitution without permission of the United States. That's how much the United States trusted the people of Utah at the time of statehood:
"The following ordinance shall be irrevocable without the consent of the United States and the people of this State . . . No inhabitant of this State shall ever be molested in person or property on account of his or her mode of religious worship; but polygamous or plural marriages are forever prohibited
Utah Constitution Article Three, Section 1.
Bigamy is a crime under Utah law (Utah Criminal Code Section 76-7-101, which includes purporting to marry another or cohabitating with another when one is married - specifically to address post-Manifesto 'polygamy,' where a party can be legally married only once. Polygamous co-habitation is illegal under the Arizona Constitution.
One might assume that the Attorneys General of the States of Utah and Arizona would have been enforcing the anti-polygamy laws - because we're not just talking in the recent past, after recent Supreme Court rulings. But they don't enforce them. And they haven't enforced them. Why? I'll let you hold your finger to the prevailing religious winds in Utah and northern Utah and take an educated guess.
Am I presuming that the laws are not enforced? No. I'm taking the Attorney General of Utah at his word. The official website of the Utah State Attorney General, Mark Shurleff, states that:
Polygamy is illegal in Utah and forbidden by the Arizona constitution. However, law enforcement agencies in both states have decided to focus on crimes within polygamous communities that involve child abuse, domestic violence and fraud.
Which means: "we're not going to prosecute any polygs. They're everywhere. They've always been everywhere. Do you actually think we meant to honor that commitment to the United States? Oh, c'mon."
I know that many people will say the government shouldn't be involved in marriage, but this situation goes further than that. It's not a case of the government staying out of religion, it's a case of the government being told not embarrass the prevailing state religion. The Attorneys General aren't going to take actions that put a spotlight on polygamy because . . .
It is the policy and practice for the Church of Jesus Christ of Latter Day Saints to excommunicate anyone involved in polygamy or who are in any polygamous organization.
~ Wilford Woodruff, 4th LDS President