Why does it lock all the topics? I don't understand why people aren't allowed to post on old topics.

Then again, I'm new, so I probably just missed something :)

Generally, no one wants to have to look back at old topics about which no one cares anymore. The current setup allows you to choose an age limit for messages that will be displayed when you go to a forum page, so if someone posts in something older, you will see that there are new messages, but not see which ones. You'd be better off starting a fresh conversation about a topic, without requiring new people to read a humongous thread from months ago to get into the loop.