Even if a shop accepts Bitcoin directly, it goes through a very expensive payment provider (let's not name them but we all know they now the one that asks for KYC too). If I can buy a coupon through LN, it's actually cheaper than using the payment processor.
Not only cheaper, but also far more secure and better for my privacy than using the aforementioned unnamed payment processor, not to mention that by refusing to use them you aren't supporting their anti-bitcoin ethos, their attacks on privacy, their cooperation with blockchain analysis, their decision to freeze or reject some payments, and so on. I'd much rather use a reputable non-privacy invading coupon or gift card seller as the middle-man than use a bitcoin payment processor which actively works against everything bitcoin stands for. In fact, I flat out won't use this payment processor at all, meaning it's either gift card or I take my business elsewhere.
That's how it should work in the ecosystem: you accept BTC, I pay with BTC.
I completely agree. However, it usually works like this: I pay with BTC, a payment processor middleman accepts BTC on your behalf, you receive fiat. I'd love more merchants to start accepting bitcoin directly or with self hosted payment solutions such as BTCPay.