I don't see anything wrong when people reply to inactive poster because the discussion isn't only for the poster, but other people that interested with the discussion can learn something and ask other questions in that thread.
In short it depends on the topic started by the Owner. And also I agree with this mate not all topics are too useful but it it depends on the main purpose of the topic. Some topics are useful so some of the time many users will reply on that thread and also those users who reply us that they will give thier ideas or learned something from other users. So if the Poster is in active then the thread he/she made will still active because of the useful topics. Anyways it won't long last forever. There's a chance that the topic will close so the users will not gonna make a reply.