I agree with you Adelaide, that *is* annoying, and rude to the OP of a thread, That was't quite what I was getting at though.
What I was getting at was threads like, say, 'The Snow On Roofs Thread' (completely made up, obviously, because I struggled for something to put here and looked out my window and saw snow on the roof). People post 'There is snow on my neighbours roof today'. 'There is snow on the roof across the street today', etc etc, until there is twenty pages talking about whose roof has snow on it. Discussion then moves to how to effectively remove snow from roofs for a few posts, until someone dives in and says 'Can we please get back to posting about snow on people's roofs?' - but without posting anything about snow on roofs themselves.
It's like they're saying 'I want to read about roofs that have snow on them. These people aren't providing that for me, so I'll chastise them instead of getting the thread back on track myself'. It's also akin to saying 'Entertain me - talk about what *I* want to hear!!!', which I find rude.
Like I say, I find it like walking up to a discussion at a party which you were not involved in and asking them to change the subject to something else, just to please you.