I believe your explanation, although technically accurate, is not a viable argument.

Let's take your example and use it else where

You say that if you get stiffed all night, you make negative money.

If a hotel has bookings and they all no show, they will make negative money, paying staff, electric while receiving no revenue... but the hotel has few no-shows and they do make money

If an airline has bookings and the pax all don't show, the flight loses money. But only some passengers no show and the flight goes on.

If a bank suddenly has everyone with drawl all their money, they wouldn't have enough to operate. But if only a few people withdraw, they still operate and make money.

IF ALL YOUR CUSTOMERS LEFT NO TIP, I agree you would lose money for that day. But how many times has that happened. I'd think it's just as the examples above. Some customers leave nothing, no show for hotels and airplanes, withdraw from the bank. But, I'm willing to bet you make umpteen times more than you've ever lost. On the average, I'd say a waiter makes more than most people would believe.