Because in real speech with real people in my real life, expressions like "Why would you do x" are nearly exclusively rhetorical and unambiguously accusatory.

Similarly, I have heard expressions like "Why would you do x" and "Why would you think y" as questions being asked with curiosity. I've also seen how my perception of their intentions has been distorted such that I would not believe they are simply trying to understand. That isn't to say the phrasing does not seem to trigger such a response (I personally think it sounds hostile in text), it is to say that I can take them at their word when they tell me they did not have that intention. I will, however, concede that the phrase seems to be more commonly said with hostile ignorance rather than humble curiosity.

But... that's kinda beside the point which I did not seem to have made clearly. The original assumption of that person's decision to write that phrase is that it is intended as an accusation to this thread's OP that their brother was actually selling heroin. They do not seem to have made that specific accusation. Recall what I originally replied to:

> You're implying that someone who didn't fight is presumably guilty

That is decidedly not what they were implying, if they were implying anything.