I've never paid for Emerald Club, so I can't be sure, but National's site says:
How To Join
To join Emerald Club for an annual fee of $50 (U.S.D.), simply complete and return the Emerald Club Membership Request, and upon processing, the benefits of this exciting program will be yours. Enjoy a whole new experience in car rentals exclusively for experienced travelers like you.
--------------------------------
Just like they give free memberships to DC members and to corporate members, they probably waive the fee for renewals to get repeat business.