Have a look at the Welcome email and PM - you'll notice the bit about being a newbie member to begin with and getting access to
chat and other things once you become a full member.
It also tells you how many posts you need to make for this automatic account upgrade to kick in ...
It's an anti-spam measure that protects us from people registering and going bonkers posting ads and links all over the place.
Cheers,
Shaun