In that vein, I think another reason for the 5-post count is to gently change the newbie mindset:
from something like "I've got to make five stupid posts when all I want to do is make a really important post as soon as I'm out of this stupid prison. What a dumb place this is that I really really want to join. They're obviously a bunch of idiots and I demand my right to join them right now or I'll hold my breath and turn blue..."
to something like "I can't think of anything useful to say right now, so I'll go and do some reading and when I have something interesting to say I'll say it and perhaps if readers find my post interesting we'll engage in conversation about it and I'll learn some stuff and... hey, I just noticed I've got more than five posts already so I'd better start to think seriously about how I can contribute to the other discussions that I'll be getting access to."
The present system isn't foolproof but it's better than many fora have. Lotta noise on the Interwebz.
If that's the goal it's not working very well because I see plenty of stupid things being posted in the other topics.
From what I can gather the newbie jail has nothing to do with anything. It's just a hazing ritual like you would see at a fraternity or something.