Retail Company Judging Based off Reward Signups

I was warned this month for having lower than normal reward signups (70% as opposed to 90%). I don’t understand how me convincing someone to join our rewards program is a direct relationship to my performance as an employee? I say the prompt they tell me to say, and add a little more like “Oh I personally use it all the time its very handy” etc., but I cant control a consumer wanting to sign up for another rewards program. My warning wasnt bad but Ive been working here for 4 or so years and its just frustrating that I had to have a meeting with my boss over something that feels entirely out of my control.