Sonic Drive-In review: won't accept coupon

This is a complaint re: Sonic #4325 located at 907 Jordan Ln. NW, Huntsville, AL 35816. I visited on the evening of 10/29/14. I made a purchase and attempted to use my Birthday Coupon for an order of medium tots, which was supposed to be free with any purchase. However, I was told by 2 separate workers that she (never specified who, but I assume the manager) does not accept these types of coupons. It was a valid coupon with an expiration date of 11/25/14, printed out through my account on the Sonic Drive-in website.

This is wrong, unreasonable and totally unfair. So, how and why is the Birthday Coupon, and all the others, offered if Sonic restaurants will not honor it/them? Why even put out the coupon to customers, then not accept it? As a loyal Sonic customer, I am extremely disappointed at this shoddy disservice.