As Andrew’s post on Judge Tauro’s ruling concerning the Defense of Marriage Act (DOMA) proves, conservatives will find a great deal of intellectual meat in the development — setting principles of federalism against a traditional understanding of marriage. I’ll have more to say on the issue in days to come, but for the moment, discussion in the comment section is definitely worth a look. Commenter Brassband offers an objection:

I haven’t had a chance to go through these opinions with any care, but based on what I’ve seen I don’t get the 10th amendment argument.
There’s nothing that compels a state to participate in Medicaid — or in a range of other federal programs.
For a state that is offended by DOMA’s application to Medicaid recipients within its borders, the solution is to reject Medicaid and provide all of the funding for that type of program from State funds, and give the benefits to whomever the State likes.
Doesn’t seem like a very well reasoned 10th amendment decision, from everything I’ve seen.

To which Andrew responds:

On the technical side, Judge Tauro’s Tenth Amendment ruling is based on a First-Circuit precedent…

In United States v. Bongiorno, the First Circuit held that “a Tenth Amendment attack on a federal statute cannot succeed without three ingredients: (1) the statute must regulate the States as States, (2) it must concern attributes of state sovereignty, and (3) it must be of such a nature that compliance with it would impair a state’s ability to structure integral operations in areas of traditional governmental functions.”

More broadly, the ruling basically says that if the states have never given away their authority in an area of governance (and, according to Bongiorno, the area is integral to the state), then the 10th Amendment makes clear that the states remain sovereign, and the Federal government cannot make its own set of rules in that area. I know that’s not usually how the 10th Amendment has been interpreted (on the few occasions that it has been), but it seems to be closer to its original meaning than is the idea that the Federal government has a broad authority to do anything it decides for itself promotes the “general welfare”, with states being allowed to opt-out of programs that are not based on more specifically enumerated powers.

It seems to me that Andrew’s explanation (and, I take it, Tauro’s) doesn’t address Brassband’s objection. The key is number 3 in the internal blockquote, which requires that a challenge on Tenth Amendment grounds involve a regulation:

… of such a nature that compliance with it would impair a state’s ability to structure integral operations in areas of traditional governmental functions.

DOMA doesn’t regulate a state’s activities as a sovereign entity. It regulates them as a dependent entity. That is, the state need only comply with the regulation to the extent that it (for example) chooses to rely on federal funds for Medicaid.
I’ve written before that I think that the spending mechanism for federal imposition of policy is corrupting of states’ sovereignty, but circumstances are much worse if the federal government is seen as merely a source of nationwide taxpayer dollars for individual states. Where conservatives typically decry the strings that come with federal dollars, it’s directly related to the regulation. This occurs, for example, when the feds offer aid for education and then dictate how individual school districts must construct their programs (or their faculties or student bodies).
In this case, even that isn’t happening. The government isn’t saying that states can only receive Medicaid funds if they define marriage along federal guidelines; it’s saying that the funds can only be distributed in ways that accord with the federal government’s understanding of marriage. Applying this to the Bongiorno rules for a Tenth Amendment challenge, one would have to argue that (again, for example) refusing Medicaid “would impair a state’s ability to structure integral operations in areas of traditional governmental functions.” That’s undoubtedly true, in a practical sense, but codifying it into law isn’t a turn of events that ought to encourage conservatives (and libertarians, much less).
The most immediate reason conservatives should be wary is that it means that Americans no longer possess a substantive say in the application of their taxdollars, when those dollars are given to the states. The secondary reason, which will perhaps prove more insidious, is that it opens up a new area in which the federal judiciary has authority to determine when taxpayers retain that right and when they don’t.