EDIT This post was a partial brain-fart, see my subsequent post for proper information.
That's strange. I suppose it's possible that they changed that with the recent panel/website upgrade. In the past you could sign up for a web-id without a hosting plan, but I can't find that link now either. Maybe I gave you a bum steer. .
By default, the $97 goes toward your hosting plan if you have one*, but you can arrange for other methods of reward payments - and I know you could participate in rewards without a hosting plan.
Of course, I've been here so damn long, and it's been ages since I've played with that side of the system, things may have changed...though just recently I know of a user that is running promo codes with no hosting plan.
Hopefully, someone who know the keystrokes for that will pipe up...I'll go look for the link(s) .