1) The etiquette isn't different for a child. Everywhere is "public" for a child.
2) The act of entering the bathroom (the same act that created the issue being discussed) wasn't carried out by Person A. It was Person B. That is why I said it was more directly the fault of Person B.
3) Etiquette isn't universal or inherent. It is due to social norms. I came from a society (or at least family) that believed knocking was as essential as covering one's mouth when coughing or not farting at the dinner table. We can all be reasonable and disagree since we aren't arguing facts.
1) If the etiquette isn't different for a child, then the example is not an entirely valid analogy, which I know isn't exactly what you were trying to make anyway. The example is a specific situation under specific circumstances.
2) It is about personal responsibility. Either you can lock the door, a 100% reliable (under the circumstances listed) primary preventative action that involves you taking responsibility for yoursef, or you can expect others to knock on the door, a secondary failsafe action that involves expecting others to be responsible for you. What could be a more appropriate barometer for measuring the correctness of an etiquette question than first determining whether each individual takes responsibility for what they can be reasonably expected to take responsibility for? To imagine that knocking is the primary prevention method and locking the secondary is backwards by any standard.
3) Etiquette isn't, strictly speaking, universal or inherent, but the principles of etiquette are derived from intuition, experience, potential consequences etc. Certain principles of etiquette have been honed to the point that they are fairly well and universally accepted, whether written or not. The best we can do is arrive at principles that are intuitively correct. In that respect, they can become more like facts in that they can be objectively superior or inferior under close examination. I can agree to disagree, but I can't abide a bad idea continuing to masquerade as a good one.
I am pretty certain I can't contribute anything new to this thread at this point. I apologize if there's any perception that I am overplaying my hand on this one. I would like to think that I am very polite to people in general, but I simply don't have it in me to be polite to bad ideas, and the idea that the universe is
more responsible for your privacy than you are for your own is a sensationally bad idea, and I don't think that is a statement that needs to be qualified as merely an opinion.
Edit: I suppose there is a dynamic of perceived victimhood re: person A. There is, thus, an element of pity that exists for the person whose privacy was violated. I submit that this is an overemphasized dynamic that adds to the confusion of answering this question in a morally "correct" way. I guess people think that the situation sucks more for person A than person B and therefore are more inclined to blame B. That is about the only way I can account for the B votes. The victimhood principle has to be secondary to the personal responsibility angle because person A is a victim of their own failure every bit as much (if not moreso) than the imagined failure of person B.