As you do not come from America, you have missed a very important part of the tipping process. America's internal Revenue Service determined a few decades ago that the average American waiter earns gratuities that equal his share of 15% of the company's annual receipts.
Then they told every waiter in America that he/she was required to pay income taxes on that amount - even if he did not actually earn it !!
To this day, every restaurant operator in America is required to report this amount to the IRS, and assist them is forcing the waiter to pay the appropriate amounts in taxes. So even if you do not tip the waiter - or tip him less that 15%, the waiter still must pay income taxes on it. In effect, he loses twice. First he loses out on the tip, and second, he pays taxes on income he did not receive.