Because black customers pay the same prices and do not require additional accommodations. It would apply if the restaurateur was required to maintain separate dining rooms for whites and blacks or additional water fountains.
Everyone pays the same prices for what they order, but they can choose to order more or less expensive foods, more or less expensive wines (or none at all), etc., and they can tip differently.
If you have more customers trying to make reservations than tables to seat them, it is economically rational to seat the customers most likely to buy your more expensive items. We decided (correctly, IMO) that it is more important for society to fight entrenched racism than to enable business owners to make that particular locally-rational economic decision.
