It makes good sense. Encourage customers to use a payment method that costs you less in fees.
Many say the discount should be proportional to the amount you save in fees, but fee cost is not the only savings. Lightning doesn't have any chargeback risk, either, which is a potential cost hanging over the head of the merchant for 60-90 days after a credit card transaction takes place, in which they still had to pay the transaction fee for the processing of the transaction, but they also lose the entire transaction amount AND are still out the goods or services they provided.