I disagree. There is no Constitutional issue nor any conflict with federal law. The general rule is that where the taxpayer has an option to do something — like the option to file jointly or married filing separately, the state court may require a party in a divorce to exercise his or her option in a particular way. The federal government doesn't care because its tax laws are not affected by what the state is doing. If your statement were true then courts could not compel a custodial parent to sign a Form 8332 to give up the benefits associated with a dependent child either. Yet they do so all the time, even though that is compelling them to do something "that has to do with federal taxes". I get that you may not like that states may do that, but as the law is now they may. No federal court decision says that they cannot. Until there is such a decision from the federal courts (which would have to end up coming from the Supreme Court since divorce cases are not originated in federal court) those states that require the joint filing are not violating the Constitution.
I agree, but will add that in the state court decisions I did review in which the state courts stated the trial court lacked the authority to compel a spouse to join in filing a joint return, they did it based on reasons of state law, not based on the federal Constitution or any federal law. So I'm not even seeing a conflict for the Supreme Court to resolve. Of course the Supreme Court could agree to hear it even without a conflict if it though the Constitutional argument compelling enough warrant its review.
Those cases are orders for years preceding the date the divorce is final. The year that the divorce is final the couple cannot file as married filing joint because they are no longer married at the end of the tax year. One of the cases I read in a state that allows the trial court to make the joint filing order the court struck the order for the year the couple got divorced noting that the court could not order them to file jointly for a year in which the Code does not allow for it.
And then face the possibility of contempt sanctions unless you can convince the U.S. Supreme Court to hear your appeal of the state court's actions and actually get the Court to accept your argument. I doubt that the Court would even grant cert to hear it.
That's the sort of factor the courts consider when deciding to make the order. None of the states require trial courts to order joint filing, they only say that that the trial courts have the power to do so.
The IRS will not accept the court order as meeting the requirements of the waiver that the Code requires (although it did prior to about 2009, as I recall). The noncustodial parent still needs the Form 8332. The IRS has never said, however, that state courts lack the power to make those orders. And such orders are routinely made in divorce/child support proceedings, as you know. If you were correct, they would lack the power to do it. Yet no federal court has ever said that state courts cannot do that in all the decades that such waivers have been used.
It is, of course, the case that the state cannot make someone violate federal tax law or require the IRS to accept a state's determination on federal tax matters. But neither of those things are going here. Where the Code gives the taxpayer an option of what to do, the court may order the taxpayer which option to take. So long as doing that complies with federal law there is no conflict with federal and the state has done nothing to harm federal government interests. The option is there; the IRS doesn't care which option the taxpayer takes so long as the rules for that option are met. The IRS has never said that state courts lack the power to make the order and, indeed, prior to the regulation change I mentioned the previous regulations expressly recognized the state court orders which it could not have done if the Constitution or federal law said that a state lacked that power. While the regulations changed, the reason for the change had nothing to do with the power of the state courts. And the same principles that allow the state to make those orders on the child dependent exemption apply to the option to file jointly.
However, if you had seen the kinds of state court orders (in divorce situations) that I have seen that blatantly violate federal law, you would realize that state courts do lots of things that violate federal law...particularly tax law.