SCOTUS holds exclusionary rule inapplicable in knock-and-announce cases

In Hudson v. Michigan, the police obtained a warrant authorizing a search for drugs and firearms at the home of Booker Hudson. They knocked on the door, waited about 3-5 seconds before entering, and discovered large quantities of drugs and loaded gun. Hudson was charged under Michigan law with unlawful drug and firearm possession. The case reached the Supremes because Hudson moved to suppress all the inculpatory evidence, arguing that the premature entry violated his Fourth Amendment rights. The Michigan Court of Appeals held that the evidence should not have been suppressed and a 5-4 majority of SCOTUS agreed. According to the Court (in an opinion authored by Scalia):

In other words, exclusion may not be premised on the mere fact that a constitutional violation was a "but-for" cause of obtaining evidence. Our cases show that but-for causality is only a necessary, not a sufficient, condition for suppression. In this case, of course, the constitutional violation of an illegal manner of entry was not a but-for cause of obtaining the evidence. Whether that preliminary misstep had occurred or not, the police would have executed the warrant they had obtained, and would have discovered the gun and drugs inside the house.

. . .

[Mapp v. Ohio and similar cases] say nothing about the appropriateness of exclusion to vindicate the interests protected by the knock-and-announce requirement. Until a valid warrant has issued, citizens are entitled to shield "their persons, houses, papers, and effects," U. S. Const., Amdt. 4, from the government's scrutiny. Exclusion of the evidence obtained by a warrantless search vindicates that entitlement. The interests protected by the knock-and-announce requirement are quite different and do not include the shielding of potential evidence from the government's eyes.

. . .

What the knock-and-announce rule has never protected, however, is one's interest in preventing the governmentfrom seeing or taking evidence described in a warrant. Since the interests that were violated in this case have nothing to do with the seizure of the evidence, the exclusionary rule is inapplicable.

The dissent suggests that the majority opinion destroyed "the strongest legal incentive to comply with the Constitution's knock-and-announce requirement" by holding the evidence need not be suppressed if the police ignore the knock-and-announce rule.