That rule only applies to posts where no topic is given. If you give it a topic (e.g., Football, Basketball, etc), you won't see that problem. If when you create a new topic, select the topic near the top right. If you don't, the topic becomes 'Uncategorized' and it will be locked after 30 minutes.
Another way to create a topic is to go into the category (Football, etc), and then create the topic. I believe it will default to that particular subject.