It is illegal to marry underage, or under any coercion, or where money is exchanged ("bride sale"), or for the purpose of any visa/immigration benefits. It's fraud to marry for monetary benefits - thus illegal. The marriage must be of the free will & consent of both parties who are of the legal age of consent, and must be a legal marriage in the country where performed (religious marriage, e.g. Islamic religious marriage) is not valid in the US. Multiple marriage (polygamy) is illegal in US (& in all but certain Islamic countries).
If the bride was sold in an Islamic country, she is by their laws "chattel" or property of the spouse. Women have no rights whatsoever. So she could not leave her husband without being arrested, and probably be stoned to death. She could legally leave her husband in any non-Islamic country, but she won't.
If she is the US, and escapes her husband, the marriage is not legal in the US & she will not be arrested for leaving him. She can legally annul the marriage since it is not valid in US. However, if she is muslim, she must expect to be killed by her father and/or brothers ("honor killing") if she leaves her husband. If she renounces islam, they will kill her for "apostasy" as islam prohibits renouncing that religion or practicing any other religion. It's not legal to do any of these so-called "honor killings," but there is no way to prevent it. So even if she leaves her husband in the US (or any other non-Islamic country), she will die. The problem is her religion, not US (or other countries') laws. No muslim girl would leave her husband - she's muslim & believes in muslim (sharia) law, and she knows what the deal is.