The government has been arguing that there shouldn't be a "knock-and-announce" rule -- and that it shouldn't apply if the cops have a warrant -- for a long time. And, when they do have a warrant, there is a case to be made for it. But even Scalia, in the recent case, noted that "The common-law principle that law enforcement officers must announce their presence and provide residents an opportunity to open the door is an ancient one," and it was written into federal statutes in 1917. The Supreme Court decided that the "knock-and-announce" rule was required by the Fourth (and Fourtheenth) Amendments only as recently as 1995 (Wilson v. Arkansas).
In practice, it rarely mattered. There are so many exceptions to the rule that any cop with two brain cells could figure out the right things to say to get around it. Things like the cops had a "reasonable suspicion" (not a very high standard) that, for example, "circumstances presen[t] a threat of physical violence," or there is "reason to believe that evidence would likely be destroyed if advance notice were given," or knocking and announcing would be "futile."
The Hudson case dealt with one of those rare instances in which the cops, for whatever reason, couldn't pull it together, so everyone -- including the State of Michigan -- agreed that the rule was violated and that the evidence was illegally seized. The only question was what to do about it. The usual remedy for the illegal seizure of evidence to keep the evidence that was illegally seized out of court (Weeks v. US (1917) and Mapp v. Ohio (1961)), which is a remedy law enforcement understands and it actually does deter police lawlessness. The Court decided the exclusion of the evidence is no longer a remedy for a violation of the "knock-and-announce" rule, which essentially eviscerates the rule.
My concern is not so much with the dismantling of the rule -- which, I think, will have a relatively small impact -- but with the ease with which the Court reversed itself on the logstanding rule that illegal searches usually require the evidence to be suppressed, and the serious absence of legal - as opposed to policy or political-- reasoning that went into the decision. That doesn't bode well for future decisions.