They recently had a promo where you could text them and they would text you back with coupons for a free sandwich, etc. All you had to do was show them your cell phone and the text. It seems that they do accept coupons from mobile devices and use technology.

I disagree with you that the cashier should have honored the coupon which you did not have with you. It is likely store policy that for this type of promotion, the customer needs the physical coupon. Maybe the franchisee sends them back in for reimbursement of some sort. Another possibility is that they need it to track each discount, as mentioned by another commenter. Showing your iPhone was your idea of eligibility, but it is apparently not their.

A lack of preparation on your part does not mean that they are required to honor the promotion. It is your fault that you did not have the coupon, not theirs.

I do not see how the employee was not empowered. Just because the employee told you no does not mean that they are rude or not empowered. At my job, I am empowered to do whatever it takes to satisfy my customers, but there are some instances where I do have to say no, such as honoring a promotion without the certificate.