And as tips are paid at the end of service, the waiter doesn't know if you'll tip well or not while serving you.
At bars on the other hand the 'corruption' is much clearer. Tip badly for a round of drinks and you'll get slow/bad service for the rest of the evening. Tip well and you'll get priority service and perhaps even free booze. Especially with spirits and mixed drinks the corruption is obvious. Tip well and the bartender will pour you a double while charging you for a single, essentially 'stealing' from his employer and selling you the stolen goods at a discount under the table.
At bars on the other hand the 'corruption' is much clearer. Tip badly for a round of drinks and you'll get slow/bad service for the rest of the evening. Tip well and you'll get priority service and perhaps even free booze. Especially with spirits and mixed drinks the corruption is obvious. Tip well and the bartender will pour you a double while charging you for a single, essentially 'stealing' from his employer and selling you the stolen goods at a discount under the table.