It's that the forum could be made more user-friendly by addressing this issue that is sure to be slowing down it's growth.
For example the post button could be made available for the newbies on all forums, but when clicked it would show a message explaining why he is not yet allowed to post there, just my 2 cents.
Never works. You can put all the info on front of everyones eyes and there's always someone who will not see it.
In a forum I run I have overly complicated password requirements: Mixed case chars, numbers and symbols must be used in addition to it being longer than 10 chars. It's written in bold red on a black background below the password fields on the registration form and the error that pops up when people fail the password requirements says the exact same thing.
Result: Every day I get emails of people complaining that they can't register for some reason they can't quite figure out... Incredible, isn't it? Should I use flashing neons?
Gotta love the internet, right? Filled to the brim with geniuses, I swear.