My job used to be the processing of coupons and the training of coupon usage to cashiers. Manufacturers will not reimuburse the store if the coupon is not handled correctly. Primarily, no store has to take coupons, for any reason whatsoever. Secondly, coupons carry no value, or severely diminished value, such as 1/100th of one cent, or some variation. Thirdly, no more than one like coupon per register receipt or transaction, also known as purchase. If I filed a reimbursement claim that had more than one lke coupon per receipt, my claim would be denied, and my store would lose the coupon amount(s), plus the handling fee(s). Fourth, coupons are not to be combined with any other offer, whether stated or not, such examples are the BOGO offers, in store % off promotions, in store coupon offers, just to name a few. This means you can have the coupon, or the offer, but not both.
The problem is not the coupons, but certain stores that have decided to take a loss just to shut up whiney 'costomers'. Customers that cost them money, should be called 'costomers'. When these certain stores process coupons incorrectly, 'costomers' expect other stores to do the same, then get mad when the proper procedures are actually followed.