I'm having a problem with this line of thinking right from the start of this.
The customer wrote a message on a business transaction document. How is that in any way considered private?
The transaction is taxed, which is a public activity.
If the bill was paid by credit card, then the customer is authorizing a third party to settle the account with the restaurant on the customer's behalf. That loan could be assessed a finance charge if the customer doesn't pay the credit back at the end of the month. That interest becomes income to the credit card company, which is also taxed, a public activity.
After all that activity involving third parties and several layers of public taxation, the customer is still crying about "privacy?"
-PJ
Wow! (No, seriously, an excellent analysis.)