The bare holding of the case is simple: if police have a warrant to search a home, and they enter in a way that violates their constitutional duty to knock first and announce themselves, the evidence turned up in the search can be used in a criminal prosecution.
The critical point is the assertion that police have a "constitutional duty to knock first and announce themselves" in the execution of a warrant. The Court accepts that reading of the Constitution. The syllabus that accompanies the Court's holding begins with this:
Detroit police executing a search warrant for narcotics and weaponsentered petitioner Hudson’s home in violation of the Fourth Amendment’s “knock-and-announce” rule.
Where is that rule found? It's not spelled out in the Fourth Amendment:
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
We have "knock-and-announce" for the reason given in Justice Thomas's majority opinion in Wilson v. Arkansas (1995):
At the time of the framing, the common law of search and seizure recognized a law enforcement officer's authority to break open the doors of a dwelling, but generally indicated that he first ought to announce his presence and authority. In this case, we hold that this common-law "knock and announce" principle forms a part of the reasonableness inquiry under the Fourth Amendment.
However, later in Justice Thomas's opinion we find this:
This is not to say, of course, that every entry must be preceded by an announcement. The Fourth Amendment's flexible requirement of reasonableness should not be read to mandate a rigid rule of announcement that ignores countervailing law enforcement interests. As even petitioner concedes, the common-law principle of announcement was never stated as an inflexible rule requiring announcement under all circumstances.
It's obvious that "knock-and-announce" is a patently absurd rule for those cases in which knocking and announcing would enable a suspect to destroy, hide, or abcond with the very items that are the subject of a search warrant. In fact, Justice Scalia's majority opinion summarizes the exceptions to "knock-and-announce."
Hudson v. Michigan is controversial mainly (solely?) because, as Justice Scalia states, "[f]rom the trial level onward, Michigan has conceded that the entry was a knock-and-announce violation." If the State of Michigan had not made that concession, Hudson would never have made it to the Supreme Court.
Here's my take: Because of Michigan's concession, the Court was bound to accept as "fact" that the entry into Hudson's house was a "knock-and-announce" violation. But the facts of the case suggest that it was not a clear-cut violation of "knock-and-announce." Again, from Justice Scalia's opinion:
Police obtained a warrant authorizing a search for drugs and firearms at the home of petitioner Booker Hudson. They discovered both. Large quantities of drugs were found, including cocaine rocks in Hudson’s pocket. A loaded gun was lodged between the cushion and armrest of the chair in which he was sitting. Hudson was chargedunder Michigan law with unlawful drug and firearm possession.
This case is before us only because of the method of entry into the house. When the police arrived to execute the warrant, they announced their presence, but waited only a short time—perhaps “three to five seconds,” App.15—before turning the knob of the unlocked front door and entering Hudson’s home. Hudson moved to suppress all the inculpatory evidence, arguing that the premature entry violated his Fourth Amendment rights. . . .
When the knock-and-announce rule does apply, it is not easy to determine precisely what officers must do. How many seconds’ wait are too few? Our “reasonable wait time” standard, see United States v. Banks, 540 U. S. 31, 41 (2003), is necessarily vague. Banks (a drug case, like this one) held that the proper measure was not how long it would take the resident to reach the door, but how long it would take to dispose of the suspected drugs—but that such a time (15 to 20 seconds in that case) would necessarily be extended when, for instance, the suspected contraband was not easily concealed. Id., at 40–41. If our ex post evaluation is subject to such calculations, it is unsurprising that, ex ante, police officers about to encounter someone who may try to harm them will be uncertain how long to wait.
Reading between the lines, the majority in Hudson v. Michigan believed that the case did not involve a "knock-and-announce" violation. But the majority could not change the fact of Michigan's concession that there was such a violation. So the majority did the next best thing; it prevented Hudson from getting off scot-free, in spite of the supposed violation. How? The majority found the "exclusionary rule" inapplicable and allowed the evidence found in Booker Hudson's home to be used against him.
By its action the majority also forestalled claims similar to Hudson's. The second-guessing by prosecutors and judges of reasonable judgments made by the police in the execution of their duties -- especially in the execution of lawful warrants -- is not a defense of liberty. Rather, it undermines liberty by making it easier for predators like Booker Hudson to elude justice, on the questionable theory that "it is better that ten guilty persons escape than that one innocent suffer."
I contend, further, that a proper reading of the Constitution would require either "knock-and-announce" or a warrant, not both. At the time of the framing, when "knock-and-announce" was accepted law, warrants were not accepted law. As William J. Stuntz writes in The Heritage Guide to the Constitution (pp. 326-9):
. . . When the Fourth Amendment was written, the sole remedy for an illegal search or seizure was a lawsuit for money damages. Government officials used warrants as a defense against such lawsuits. Today a warrant seems the police officer's foe -- one more hoop to jump through -- but at the time of the Founding it was the constable's friend, a legal defense against any subsequent claim. Thus it was perfectly reasonable to specify limits on warrants (probable cause, particular description of the places to be search and the things to be seized) but never to require their use.
Hudson served justice, while remaining true to the original meaning of the Constitution.