That presumes first class (or at least credible) payroll management, which, while there are probably instances of in the Restaurant Industry, is less common than one might hope. In practice, waitresses are paid their minimum server wage and are then on their own to scrounge up tips to make up the difference. For each case where we can find where a restaurant actually made up the difference in missing tips, we will likely also find several examples of where it doesn't happen.

And, while that "$7.25/hour" is OK in some place like Terre Haute, Indiana, It's destitute poverty in a lot of cities, so the societally agreed convention is that in order to live/pay groceries/utilities/bus-fare/etc... tips are essential.

It's reasonable to assume that when a serving person in the United Stated doesn't receive a tip of 18-20%, then that money is resulting in a reduction of expected baseline salary.