The reason you can't delete topics is because SMF doesn't differentiate between topics with just your post and topics with other people's posts. It's not particularly swell to delete others posts just because you made the topic. You can normally delete your own posts, but SMF doesn't allow you to delete your post if it's the first in a topic, and doesn't differentiate that from topics with only one post. So long story short, it's a limitation of SMF.
As for why Arch can delete it, it's the moderator status, which allows deletion of other people's posts. At any rate, if you want a post of yours removed, you can always edit it and blank the post, but I'm of the personal opinion it's a good idea not to remove old posts, as it fragments replies. If it's absolutely necessary to delete a post for some reason, message any moderator or admin, but editing should cover pretty much everything except accidental topic creation.
