All permissions are configurable. For general purposes, it is set up so that once a discussion is started, it runs as long as people think the subject is interesting enough to talk about. If the original poster would close it for whatever reason, others would need to start a new thread to continue the discussion and that would be messy and annoying.

The BST is a different matter. There it was decided to give people the ability to close their own threads, because they decide whether something is still for sale or not (the item is retracted or sold). And by allowing people to close their sale threads, we don't get the situation where people keep saying 'nice score' etc. That would continuously bump sold items to the first page, swamp actual available items to next pages, and make it difficult to see from the outside whether an item is sold or not.