I like to think of the verbal, face-to-face conversational equivalent, like if a bunch of us were sitting around talking about something. Only it has to be a little more structured, since it's a thread on topic X--like we've all gotten together to discuss our new kittens or something. If someone said, out loud, "Could we please get back to the main topic?" I wouldn't necessarily think it was rude, but I would think it was kind of extreme--I personally would save that direct wording for when things had gotten really derailed, and my earlier efforts to redirect hadn't worked.
Personally, I prefer to bring a conversation/thread back on track by saying/posting something on topic, like a new story or observation that's in line with the subject of the thread. Then hopefully other people do the same, and things get back on track without anyone feeling scolded or commanded. But, I must admit it's not 100% guaranteed to work, and sometimes it can be hard to think of something new and meaningful to contribute on the spot.
Also, I think the threat of having a thread locked, and then not knowing if you can start a new one on that same topic anytime soon, kind of hangs over (some) people a bit--they're really enjoying the thread and they would hate to see it go away because it was getting off-track, more like the natural flow of conversation than a focused discussion. So maybe they feel like they have to step in a bit more strongly--not necessarily like, "You are not posting what I want to read," more like, "You guys, we're gonna get busted! And I really like this conversation."