They did have this in their ReadMe
"Note that an installed payment provider (like PayPal or CCBill) is required to receive money for Paid Memberships"
But i totally agree they should have been much clearer and specific and also should have stated this on the front page of the plugin in plain specific english as they did do on another one of their plugins. I dont think it was intentionally but probably just someone in a hurry. And yes it does need to corrected.