The 10% off is a rebate, and like discounts is a way to entice people to buy.
I'm thinking of the case when an owner who does not work in the restaurant eats there. Say someone owns a chain of fast food restaurants, or even just one fast food or fancy restaurant, but they just show up to get food, and don't normally work there. The free food is a perk for them, and is technically taxable income if they don't pay for it. If they pay for it and don't deduct it, then it's like any other customer, and it's not taxable income to them.