The website description follows the guidelines from Stack Exchange, which strives to create and support expert communities. They say:
We're creating sites for experts.
Your site should focus on pilots, not passengers; lawyers, not lawbreakers; professional photographers, not Uncle Marv with his digital point-n-shoot.
If the site can attract the experts, the amateur enthusiasts will follow!
So, the idea is that "just curious" people will ask anyway, because they are curious: they don't need an invitation. On the other hand, when a site gets dominated by "just curious" category early on, Stack Exchange may well shut it down (recent examples: Gamification, Deep Web, Artificial Intelligence, etc).