Sales people work on commission typically, and that system is absolutely not harmed by that on average. It's enhanced dramatically by it. A tip is a commission system that encourages the same way a traditional sales commission pay system does, including rewarding up-selling and efficiency.
The difference between earning 10% and 20% on average for tips can make a very large difference for a waiter. At a good restaurant that can add up to anywhere from $15k to $30k per year in difference. In the US, waiters can often make more than their managers because of the tip system.
The incentive is to earn as high a percentage as possible by doing as good of a job as possible. Everyone should not get paid fairly if they suck at their job and treat people poorly.
Waiters also have an opportunity to earn even more money by up-selling when it makes sense.