This is a K9 decision, but the court's reasoning has some pretty obvious implications that go well beyond the use of drug dogs. This is one of those decisions that every officer needs to familiarize themselves with.
Police received an unverified tip that there was marijuana in Jardines' house. The detective working the case conducted surveillance on the house for a whopping fifteen minutes, saw no activity, and then approached the front door of the house with a K9 and his handler. The K9 alerted on the front door, and the officers left. They returned later with a search warrant. They found a marijuana grow in the house, and Jardines was arrested.
The trial court suppressed the marijuana, ruling that the dog sniff was an unreasonable search and that the warrant was invalid because the police obtained it relying on evidence from said unreasonable search. This was appealed all the way to the Supreme Court.
There are a couple of different ways that the courts decide whether or not a search has occurred. The original test (which was recently reaffirmed in Jones) is the trespass test: if whatever the police are doing involves physically trespassing on someone else's property, then it is a search. More recently, Katz created the reasonable expectation of privacy test. Most cops are very familiar with that one: if a person can reasonably expect something to be private (like a phone call), then a search occurs whenever the police violate that expectation of privacy. The important note is that the reasonable expectation of privacy test did not replace the trespass test, it simply added to it.
The court explained that the curtilage is part of the home for Fourth Amendment purposes, and that laws regarding trespass establish that no one may set foot upon another's property without the owner's permission. On the other hand, our culture has a sort of implied invitation to the front door. Anyone can walk up to the front door of a house, promptly knock, wait a while to be received, and then leave (unless they are invited to stay). This implied invitation is limited in terms of the areas that a visitor can go; you wouldn't expect someone to knock on your back door, hang out by your bedroom window, or just open the door and let himself in. But this implied invitation is also limited in terms of purpose: just because we traditionally allow strangers to knock on our door doesn't mean that we would normally let someone run a metal detector over our front yard or run a drug detecting dog on our front door.
And that was the deciding factor in this case: the police went way beyond the scope of what an average person would be entitled to do while physically intruding on the property of another. This wasn't a case of knocking on the door to try to talk to Jardines, this was a search of Jardines' home which was conducted without a warrant, and without probable cause or exigent circumstances.
The prosecution tried to argue that the dog sniff is not a search because there is no legitimate privacy interest in contraband (this reasoning has been used to justify dog sniffs of cars and suspicious packages). But the court merely pointed out that this case wasn't decided on the basis of Jardines' expectation of privacy. When the police use dogs to sniff cars and suspicious packages, they are doing so without physically intruding on the defendant's property. In this case, they were very clearly on the defendant's property. That makes this a search, and since there was no legal basis for a search, the suppression of the evidence was affirmed.