Police in Michigan had a warrant to search Hudson's house for guns and drugs. They knocked and announced, waited just a few seconds, and then opened the unlocked door and conducted their search. They found what they were looking for, and a lot of it.
Hudson tried to get the evidence suppressed, arguing that opening the door so quickly violated the knock and announce requirement of the warrant and therefore his fourth amendment rights. That didn't work (the Michigan court of appeals recognized that there was a knock and announce violation, but held that suppression was not the proper remedy), and Hudson was convicted. He appealed.
The US Supreme Court described some reasons for the existence of a knock and announce requirement: it gives suspects a chance to comply with the search warrant, protects life and limb by preventing suspects from acting violently in mistaken self defense, and allows suspects to prevent the destruction of their property caused by a forcible entry. The court also described some exceptions to the knock and announce rule: the rule is suspended if the police have reasonable suspicion to believe that they will encounter violent resistance, that a warning would lead to the destruction of evidence, or that knocking and announcing would be futile.
So the point of a knock and announce requirement has absolutely nothing to do with preventing the government from searching or from discovering evidence. Therefore, the court ruled that the evidence should not be suppressed because suppressing evidence wouldn't protect the rights that are violated by failure to knock and announce. Also, the cost to society of applying the exclusionary rule would be too high, and the benefit to society of using such a harsh measure to ensure that the police wait a moment longer before making entry with a warrant would be negligible.
Hudson still had the option to sue the police civilly for the knock and announce violation, but whether or not he would win there the violation didn't affect the criminal case.