Imagine two restaurants. One has prices 15% higher than the other, and the owner pays this 15% to his wait staff in the form of higher wages. The other has lower prices, but the average customer gifts 15% to their waiter.
Clearly, in the first restaurant, the 15% the wait staff receives is taxable income. It is traditional salary. What legitimate, economic justification is their for treating the second restaurant any differently?
Imagine a grocery store in a small town that offered long-time customers a "pay nothing" option but made it clear that they'd be subject to social ostracism and no longer welcome in the store if they didn't gift 85% of the usual cost of the items. The customers would save on sales tax and the grocer would argue that all that money was gifts, not income.
Of course this doesn't work. The IRS, and the laws, don't care very much about what you call things. They care about the underlying economic reality. If the money was part of the payment for the services rendered, regardless of how it was delivered, what the parties called it, or whether the obligation to pay was legal or social, it's still a payment for the service and it's still taxable.
You would have to be able to argue to the IRS that it really was a gift and wasn't any form of payment for the service received. Otherwise, it's just a scheme to evade taxes.