Those are your permissions and yes, it is true, you can edit your post within 30 min. after publishing. I'm sure that if you refreshed the page after 30 min. you wouldn't see the link underneath the post that read "edit post". If you do (after this 30 min. time frame) then we will have to look into it because it is not supposed to work that way.
As annoying as this can be, it is a security feature and definately an anti-spam need. Many spammers post something apparently innoncent to edit their posts later on and add links and advertisments knowing that the post has already gone through.
Although the 30 min. limit is mentioned in the FAQs/Rules of the forum (http://www.sitepoint.com/forums/faq.php?faq=etiquette#faq_editingposts), it is obvious that we need to think of something that reminds users that their editing capabilities are just for limited time as the rules are often forgotten and some people don't even read them.
You can still contact a staff member and ask them to edit whatever text you want to change after that period.
I'm sorry for the inconvenience that it caused to you, though.