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.