I’m a cashier and I have a question re taxes

I’m a cashier at pretty well-known grocery chain and I have a question/story about manufacturer coupons and taxes.

A man came in with a “free bag of chips” coupon, and it didn’t scan so I called my supervisor (I’m new) and she entered it in manually. The fine text said that the coupon was applied to the retail value before taxes, so my sprvisor entered it and it left fifty-something cents in taxes. The customer (who said he was an accountant) said that he shouldn’t pay taxes on something that’s free, which makes sense, but my supervisor was insistent that it’s how these coupons are always applied.

I can understand the supervisors / stores perspective that someone needs to pay the taxes, and the manufacturer clearly said that they aren’t, but it still feels weird to charge someone taxes on a free item. I’ll post a picture of the coupon on my profile and link it here. It doesn’t matter now, the customer paid the taxes in the end, but I’d like to hear what everyone here thinks.

Edit: here