UI button placement aside for a second, don’t you think users realise when they have replied in public given their response ends up in the public stream, as it were? It’s not like there’s any sign their reply has gone into a private space for them and the person they have responded to.
I think they do realise and think that it’s OK. I agree with you that it is not always OK, at least not frequently. Odd asides are fine, but not when they start to take the whole discussion off-course. And that’s where the moderation comes in …
But this is definitely an issue worth tackling because it can take up a whole load of mod time and if there is any UI solution then I’m all in favour.