I just saw this, and it's more to do with Mexico than with the US. Mexican law pretty much always gives kids to the mom if there is a divorce or custody battle.
I'm not convinced that one has anything to do the with the other but you may be right. Regardless of who Mexico gives custody to the US can file international abduction charges against the abductor and request extradition. The FBI may hesitate to do so since it is scared Mexico will not honor the extradition request and create a diplomatic mess (which would be worse than the collateral damage of a parent never seeing their child again presumably).
Mexican law, in most states,
automatically gives custody to the mother during a divorce if the child is under 7 years old. The legal presumption is that the mother is the more fit parent, however, there are exceptions in that, if it can be demonstrated that the mother is unfit, for whatever reason, the father may be granted primary custody. That said, Hague cases are not custody cases and a return order under the Hague Convention does not amount to a custody decision only a tacit acceptance that the child should be returned to the jurisdiction in which custody should be decided. Additionally child abduction and/or retention is a serious crime in most of Mexico under the laws of most states and the federal penal code. These laws do not apply for crimes that occur in foreign countries because the penal code in Mexico, with some few exceptions, only applies for crimes committed on Mexican soil.
In theory Mexican law does allow for extradition of its citizens when the crime for which extradition is requested is actually considered a crime in Mexico (and as I stated above abduction/retention is a crime in Mexico) although I don't know of a single case where the US has requested extradition of a Mexican mother for abduction and it very may well be because they don't think Mexico will honor it based on their preference for Mexican mothers.
I do know of a case where the FBI has at least issued a federal warrant for kidnapping against a Mexican mother though.. even if they didn't request extradition:
http://www.fbi.gov/wanted/parent/vera_a.htmKind of stupid to file "international kidnapping charges" and not request extradition of the kidnapper though...