The Fourth Amendment warrant requirement is of course riddled with exceptions.  One longstanding rule, for example, is that no warrant is needed to conduct certain searches made incident to a lawful arrest.  This "search incident" doctrine is the product of a relatively straightforward calculus:  Following a lawful arrest, the state's interests in officer safety and preventing the destruction of evidence outweigh the privacy interests of an arrestee.  Just yesterday, in Virginia v. Moore, the Supreme Court clarified the meaning of the doctrinal term "lawful."  This question is more interesting than one might think. 

The debate in Moore basically centered on whether, for the purposes of the "search incident" doctrine, a "lawful" arrest is one consistent with the Fourth Amendment, or, on the other hand, one consistent with constitutional and positive law.  It is not surprising that in our federalist system the two are not always one and the same.  For example, an arrest for a minor criminal offense is "constitutional" so long as probable cause exists.  See, e.g., Atwater v. City of Lago Vista (upholding the famed arrest of a soccer mom for not wearing a seatbelt).  Atwater aside, states are of course free to curtail arrest powers beyond the limits imposed by the Fourth Amendment.  Such extra-constitutional limitations include prohibiting arrests for certain types of offenses and directing officers to follow specific guidelines during arrests.  It is not hard to see how an arrest could be "constitutional" all the while being contrary to state law.

This brings us to Moore, which addressed the following question:  When an arrest falls within this extra-constitutional interspace (i.e., constitutional but unlawful under state law), does an officer's search incident to that arrest violate the Fourth Amendment? 

The Court unanimously resolved this in the negative, holding that the term "lawful" means nothing other than "constitutional."  Put simply, a search is constitutional whenever it is incident to a constitutional arrest--even if the arrest is violative of state law.  This result was not unexpected, as even the more liberal justices seemed wary of Moore's claim at oral argument.  (Justice Souter:  "If the Fourth amendment says it's OK to arrest, then why shouldn't the Fourth Amendment say it's OK for the officer to protect himself.").

Despite my having some reservations about the Court's interpretation of its precedent in Di Re (see Orin Kerr's post here ), the Moore opinion is, as a matter of policy, undoubtedly a good one.  This is, in large part, because it ensures a tax-free zone for the future development of search and seizure law at the state level. 

Let me explain.   

The Fourth Amendment provides an absolute floor on search and seizure rights.  All observers, no matter their judicial philosophy, envision that the states, which are well-positioned to gauge the particular privacy and enforcement interests of their citizens, are free to enact extra-constitutional regulations to protect privacy rights beyond those guaranteed by the Fourth Amendment.     

Extra-constitutional regulations are more likely to be enacted, all things being equal, by legislatures with unfettered authority to select remedies (e.g., exclusion, civil liability, administrative sanctions, or, for that matter, no remedy at all).  If the Court had, as Moore argued, pegged the "search incident" doctrine to state law rather than constitutional law, the Fourth Amendment (which is bound to the costly remedy of exclusion) would effectively impose an "exclusion tax" on those well-meaning legislatures that opt to enact extra-constitutional search and seizure regulations.  One can easily envision a bloc of legislators which (1) seeks to curtail arrest powers in a particular way, (2) is willing to create a remedy of administrative sanctions or civil liability, but (3) is unwilling to create a remedy of exclusion.  Under such circumstances, the state government would be willing to "produce" the extra-constitutional regulations at natural costs--but not when saddled with a constitutionally-imposed exclusion tax. 

This illustrates that when the Court commandeers exclusion as the remedy for the violations of extra-constitutional search and seizure regulations, some state legislatures will rationally respond by simply repealing existing regulations and/or refusing to enact future regulations.  The all-or-nothing approach proposed by Moore, if adopted, would have likely resulted in weaker regulation of law enforcement, and, from a more theoretical perspective, an undue obstruction of the natural development of search and seizure law.  Thankfully, the Court decided otherwise, concluding wisely that the Fourth Amendment imposes no exclusion tax on state legislatures.