Basically your suggestion implicates allowing users to moderate their own threads. Blogs and groups have better case for this because their typical purposes are, respectively, providing a personal soapbox and an archived way to chat/talk. Forum threads are much less personal and not designed for chat at all, rather a public medium where a subject is presented and following posts build upon it. Arguments against locking own threads relate to this, as well as the various reasons why posts would get deleted. Admins delete posts/threads if content breaches terms of use. It could become very petty were users granted the ability to patrol their own threads; we expect users to delete their own posts when necessary (e.g. double post), but even that doesn't happen often.