why Domino's shouldn't just charge less when they can afford to.
Ideally they would charge less, but people are willing to pay the higher prices, so they charge the higher prices. We live in a capitalist society, and they'll increase the prices as high as the market can bear.
A lot of companies increase their prices but have coupons that bring the price down. Easy way for them to make more money, as people that like the product will continue to buy it, and not every customer will use the coupons.