@julian If OP is deleted, the topic surely becomes null/void. Having said this though, I can see why this is default behaviour, as if you've got a large topic going on, and OP suddenly decides to delete first post, that entire topic is lost along with all posts. Which seems a little blunt if there's good discussion going on.
Maybe not allowing you to delete the first post would be a better idea, or rather than actually deleting it, it flags it for moderator attention (user wants to delete topic) or something. Probably a little overly complicated though for such a small task.