We have a number of options of what we can do with posts that are tangential or off-topic to another person's thread. We generally do not leave them alone, because they are disrespectful to the original poster by taking the thread off-topic or into a direction that, unless the OP consents, are not in keeping with the OP's subject matter.

Typically when it's been brought to our attention, we will remove the post(s) if they are against the community guidelines (e.g., start to call out an individual for personal attack).

Or we move the post(s) into their own thread, typically within the same forum. If the post(s) are not appropriate to that forum, they will be moved to the forum they are most appropriate here.

Part of our roles in the community support team is to curate the community, ensuring that people are posting the right topics to the right forums. If we didn't, we might just as well have one big single forum for everyone's topic. People don't usually mean to post off-topic to a thread, or to the wrong forum, but it's a part of our jobs to find the best homes for topics when they do. You'll see this all the time when you see "Moved" next to a title or thread, where we've moved an entire thread to a more appropriate forum.

Yes, when a topic is moved out of a thread into its own thread, we typically notify the poster(s) that's been done. Sometimes this isn't done, and I apologize for the oversight.

I'm not sure about the "fair" question -- we do a lot of things in a community for the community's overall good and smooth functioning that would readily be uncharacterized as "unfair" by the member who's been impacted by our action. I would say we try and be sensitive to individual members' needs, but have to take into account the bigger needs and health of the community too when making decisions about posts, threads, and off-topic posts.

DocJohn