The business model works like this:

If you don't have affiliates send traffic to you, very few of their traffic signups will find your website and join your site on their own.

So...

With affiliates...
$30 x 50% x 1000 signups = $15,000 in your pocket

Without affiliates...
$30 x 100% x 1000 signups x only 2% of those would have found you = $600

You can argue that more than 2% of those members that affiliates sent you would have eventually found you on their own. But even if you bump that to 10%, that's still only $3000, far less than the $15,000 you make with affiliates.

Simply put: If you offer 50% payout to affiliates, then 50% of those members they send you would have to found your site and joined anyway to break even.

This is a bit of an oversimplification, but pretty much holds true.

--Aaron