Sometimes. Sometimes they don't bother until they actually encounter somebody, which may be too late. Sometimes throwing flashbangs and yelling "POLICE!" causes people to scatter, react in self-defense, or retaliate.
I think it depends on the scene itself. They're obviously going to want to delay the moment when those inside the house realize that they are there as long as possible. Usually, this occurs at the point of entry, since they have to make noise to batter/break/whatever their way in. At that point, those in the house know that someone
has just broken in. Surprise is gone, so you have to move fast and make a lot of noise, and there's no harm in shouting "Police! Warrant!" at that time (and a lot of potential help). If they're able to enter silently (say because the back door is unlocked), they may wait until they encounter someone. Which, admittedly, can result in problems depending on *how* that encounter occurs.
In the case mentioned earlier, a kind of worse case scenario occurred. Back door is open. They been told there's an intruder in the house. They sneak in trying to find the intruder. Then they run into a dog. Scene devolves from there. Doubly so since there was no intruder.
Here's the thing though. What if there was an intruder? What if said intruder was in the house, threatening the homeowner with a weapon? If you were the homeowner, would you want the police to declare their presence at the open back door? Or sneak around until they find you, and take the intruder by surprise where he has less chance of killing you or holding you hostage? Again, as I'm sure your aware, that sort of scenario does play out. And in that particular case, they made the right call entering silently. It turned out very badly in this case, but in most cases, it would be the right thing to do in that situation, and of course, they have no way to know which is which at the time. They are forced to play the odds and "do nothing at all" isn't a choice available to them.
Look, I'm not a cop-hater. My uncle was a D.A.R.E. officer. His two sons are sherriff's deputies. They've been working some major drug operations in central Ohio lately. I appreciate police officers. I just disagree with some methods and policies. Particularly ones that get civilians and police killed.
I agree. I think that one of the biggest problems many officers have is that they get a sort of situational tunnel vision, where they fail to re-assess their initial assumptions as new information presents itself. This can lead to semi-humorous hilarity as with an encounter I had with a particular police officer on a dark and stormy night, but it can sometimes lead to tragedy. In the case mentioned above, the presence of the dog, previously silent, but presumably taking some action deemed to be aggressive towards the officers upon entry, should have clued them in that there might not be an intruder in the house at all. The dog should have been barking or growling (or dead) already, not just now reacting to the police when they entered. But in all probability, the initial belief that an intruder may be in the house prevailed in their minds anyway. Which lead them down a tragic path.
Same deal with the cop shooting the undercover cop. He sees what he believes is a drug dealer in a car (cause that's what the undercover guy is pretending to be, right?). He assumes he *must* therefore be a target of the raid and opens the door, but then the target behaves in an odd manner (as an undercover officer would do when one of the officers on the raid he's coordinating suddenly yanks open the door and starts shouting at him and pointing a gun). He certainly didn't react as a drug dealer would, but perhaps more aggressively, maybe attempting to correct the officers mistake, but not getting through to him because in the heat of the moment, he knows this is a drug dealer in the car. Maybe he's holding something in his hand (a walkie talkie that is being used to trigger the raid perhaps?), but again, in his mindset, he doesn't see an object in the hand that fails to match his expectations of what a drug dealer in a car would be holding. He sees a gun. Something. Who knows? Adrenaline is pumping, and there's something "wrong" with the guy in front of him. He's in fight or flight mode, and his training teaches him to choose fight. He starts shooting. The undercover starts yelling at him to stop, perhaps attempting to move away, maybe still holding that unusual object in his hand that the officer has now (in his mind) identified as a gun. Perhaps waving it in front of him, trying to explain who he is and not understanding why the officer is still firing at him.
Speculation, of course. The full details aren't clear. But you can see how these sorts of tragic results can occur as a result of seemingly simple choices and thought processes that turn out to be wrong, but by being wrong, actually set up confusion in the officers mind, that results in a spiral of bad actions. The human mind is interesting in that it fills the gaps of what we see with what we assume we should be seeing. A useful trait, most of the time, like when we see rustling grass and a bit of color and fill in the lion that's coming to get us. But sometimes, it's can cause us to see things that just aren't there (or fail to see or properly identify something that is). I think that this is definitely one area that officers could be better trained to identify and recognize. But at the end of the day, it's never going to be perfect. But you could at least train them to better recognize when they've entered into that "I'm not sure exactly what I'm seeing" state, and maybe reduce the number of such tragedies a bit.