都无聊得去逛 r/china 了

Tipping was a custom invented by employers as a way to justify paying lower wages. Basically employers can say I don't need to give you a living wage because you can depend on the customers for charity.
Also don't forget it places pressure on the rest of society who require taxis as part of the transportation network. Next time that single mother in the rain is late for her 900rmb a month job can't get the taxi because the group of rich foreigners is more likely to tip.

via