Despite the "welter of tests" that have developed among the circuit and state courts on this issue, Justice Alito writing for the majority reasons the appropriate standard "follows directly and clearly" from established Fourth Amendment law, and in less than two pages of analysis arrives at the answer:  "Where, as here, the police did not create the exigency by engaging or threatening to engage in conduct that violates the Fourth Amendment, warrantless entry to prevent the destruction of evidence is reasonable and thus allowed."  (footnote omitted)  

The majority of the opinion is devoted to the inappropriateness of the alternative police-manufactured exigency tests.

A standard that looks to the "bad faith" of the police officer to evade the warrant requirement is easily disposed of as "fundamentally inconsistent with [the Court's] Fourth Amendment jurisprudence."  The Court has consistently rejected a subjective approach to Fourth Amendment questions, instead taking the view that "evenhanded law enforcement is best achieved by the application of objective standards of conduct."  (quoting Horton v. California.)  (See also Whren v. United States ["Subjective intentions play no role in ordinary, probable cause Fourth Amendment analysis"].)

The Court also rejected the unsettlingly broad "reasonably foreseeable" test, which is based on whether the exigency was a reasonably foreseeable result of the investigative methods employed by police.  That test, said the court, would "introduce an unacceptable degree of unpredictability" into law enforcement efforts and risks interfering with the quick decision-making process police encounter in the field.

A standard dependent on whether the police had probable cause and time to obtain a warrant prior to entry of the home "unjustifiably interferes with legitimate law enforcement strategies," given the numerous reasons an officer might proceed with consensual investigative methods and put off seeking judicial approval.  Penalizing police for failing to seek a warrant as soon as possible "imposes a duty that is nowhere to be found in the Constitution." 

A test centered on whether the officer complied with standard or good investigative tactics was simply too vague.  Judges, rather than law enforcement officials, would have the opportunity to evaluate what is and is not "good" investigative tactics. 

Lastly, a standard focusing on whether police "engage[d] in conduct that would cause a reasonable person to believe that entry is imminent and inevitable," proffered by Petitioner Hollis King, is a "nebulous and impractical test" the Fourth Amendment does not require.  Qualities that would signal such an entry, such as a loud tone of voice and a forceful knock, are sometimes necessary for safety reasons and to give citizens an opportunity to refuse interaction with the police.  Police would have no way of knowing how loudly they may knock or announce their presence, and "the ability of law enforcement officers to respond to an exigency cannot turn on such subtleties."

As Kent pointed out, this very pro-law-enforcement decision is likely to raise eyebrows.  After all, the idea that a police officer can detect the smell of marijuana, knock on a nearby door, and bust into the home after hearing people move about inside (the basic facts of this case) might rub wrong even the most tough-on-crime believers.  But I suspect that the real issue critics have with this scenario is not the propriety of the officer's act of knocking on the door, but whether these borderline circumstances rise to the level of "exigent circumstances" justifying a warrantless entry.  That issue, however, about which the Kentucky court expressed concern, was not before the Supreme Court in this case.  The existence of exigency was assumed.

The opinion by Justice Ginsberg, the lone dissenter, also appears to be largely an attack on the adequacy of the exigency.  "How 'secure' do our homes remain if police, armed with no warrant, can pound on doors at will and, on hearing sounds indicative of things moving, forcibly enter and search for evidence of unlawful activity?" 

But the lawfulness of a police officer knocking on a person's door without any level of suspicion is undisputed.  It is well-established that police can, and are even encouraged to, investigate via such "consensual encounters."  Any dispute over the lawfulness of a police officer entering the home based on mere noises from within strikes at the presence of an emergency, not the validity of the emergency.