The second (and related) review question I wanted to address in class yesterday was why the federal preemption of state law (by a federal statute) does not constitute a "commandeering" of the states in violation of New York and Printz.
Here is why it is potentially a problem. Recall that not all commands that Congress issues to states are unconstitutional. Indeed, the Court in New York and Printz was careful to distinguish Garcia, where the Court upheld the application of the FLSA's maximum-hour and minimum-wage provisions to the states. We spent some time (about six weeks ago) discussing how Garcia was distinguishable. First, the FLSA was "generally applicable": it applied not just to the states (or their subdivisions), but also to private employers. Second, and more fundamentally, at issue in New York and Printz were laws that regulated the states in their capacity to regulate or govern their won citizens, whereas the FLSA regulates the states in a proprietary capacity, as objects of regulation.
Notice that these two grounds, though, fail to distinguish preemption. Preemption is not "generally applicable"; rather, it is directed at states alone, as private parties are not capable of enacting laws or regulations. More fundamentally, preemption is directed at states in their capacity of governors of their own citizens. A federal statute that preempts state law is directly commanding the state as to how it is to regulate or govern its own residents. (E.g., "State, you shall not require any automobile manufacturer or seller to equip any particular car model with airbags.")
So how are the two doctrines compatible? The key is the difference between an affirmative command (as in New York) and a prohibition (preemption). The anti-commandeering principle forbids Congress from ordering states or their political subdivisions to take affirmative action to govern or regulate in a particular way. But it does not forbid Congress, in its regulation, to forbid states from regulating in a particular way (or perhaps in a particular field at all). Of course, Congress must be acting within its enumerated powers in enacting the relevant statute. But assuming it is, effectively ordering a state not to regulate is not a forbidden "commandeering."
This is how these different doctrines--concerning enumerated powers, commandeering, and preemption--all fit together.