The whole thing is bloody ridiculous to be able to delete a post. Just create a delete button so that members don't have to be posting tumultuous amount of posts, trying to find out on how to delete a post. Why make things more complicated than they are?
The reason for it being as it is was explained by Doctor Q earlier in this thread:
...we've gotten burned when we allowed the full membership to delete their own posts, because they would delete an entire thread that they had started (by deleting the first post) if they didn't like the way the thread was going, causing everyone else to lose their posts in that thread. So the moderators delete posts on request but make sure an entire thread won't be lost.
And it's in the FAQ, in this section.