Just looked at their join page
http://wowgirls.com/join.html
It lets you choose between recurring and non-recurring. BUT there's no mention of any discount for recurring.
When you proceed to the CCBill actual form, you are just shown the option you have chosen in the prejoin (and you can't change it unless you go back to the previous page)
So my hunch would be: most people just choose "non recurring" in the prejoin. They are never told, in their flow, that there's a discount for recurring.
To me it looks like one of those tiny blunders that can cost very dearly to a business.