Both the customer and the owner benefit because it's tax free -- cheaper for the customer, unreported income from the owners.
Doesn't work that way with income tax, does it? The employer wants the deduction -- he has no incentive to cheat for the benefit of the employee. If it's a buyer, the buyer wants the fulll basis, not a lower one. It's this contending interests that make it hard to cheat at income taxes, easy to cheat at sales taxes. So ironically a national sales tax would require a bigger IRS.