Jo DeDera, that is almost word for word what i was thinking.
I might not *like* being told to look at my past behaviors - it might sting a bit - but there is probably a good reason for it. I don't see that as being in the same vein as "Your mother is toxic!" or "Your boyfriend is controlling!"
And in any given situation, we're only working with the OP. They're the only one in the story seeking advice, so it makes sense to ask for a little self-reflection if something seems weird or people around them are consistently acting hurtfully. Even if someone else is clearly more at fault, we only have the power to advise the OP. If asking for that kind of information is off limits, then it does just turn into a sympathetic chorus in the thread.
But I totally agree that any kind of diagnosing or extreme labeling (abusive, toxic, controlling, evil) or treatment suggestions should be reported and shut down. Especially when it's based off one isolated incident. That always seems bizarre to me.