I had that happen to me! The funny thing was I didn't need to speak up, my hubby did for me. He looked at the girl and said "Don't you think she has enough real curl, why would she want you to put fake curls in her hair?" I laughed and we walked away, leaving her standing there with her mouth open!
I also had my hair straight one day, and had them ask me if I would let them curl it. Umm, no. It took me forever to get it straight, why would I want you to curl it back up? I can't stand those booths!