@peter Because you're deleting the first post of a topic, not the topic itself. Both have very different outcomes if used.
If someone deleted the first post in a topic with 100 replies, it shouldn't delete the entire topic, just that one post. It just so happens that, if the post is the only post in the topic, the topic will be blank.
Ideally, deleting the only post in a topic should remove that topic, but as soon as a topic has a reply, it should only delete that post. This was discussed a few months back on Github if I recall, don't think we came to a real solution.