It's easier to jump in and more free-form if you can change the topic whenever you want or whenever you feel appropriate, or just post something and if the topic changes, it will, but if it doesn't, oh well.
(This is probably going to get deleted for being off-topic. Bleh.)
As Audy said, if we didn't have the rule then the conversation would get really disrupted and this'd end up turning into more a "Post Your Thoughts Right Now!" thread than anything else. The fact that the topic does change in this thread kinda goes to show that it's free-form, though. I definitely get where you're coming from and it's unfortunate that you find it hard to jump into the conversations here but it's for the best that we don't allow random topic changes.
From what I remember having joined like... a club, all I had to do was fill in a short form with my name, what I liked in particular about the subject the club discusses, and a couple of other things which idr now and that was it. I guess, if you only see the novelty of joining a club, then just think of it as a really big discussion rather than a club. Maybe you'll find you get more out of it approaching it that way.