Specifically, the company is providing a service which depends on charity of the people receiving it to continue. Remember that charity is supposed to be volunt

Specifically, the company is providing a service which depends on charity of the people receiving it to continue. Remember that charity is supposed to be voluntary. This fact is not documented anywhere. "Customary" is code word for non transparent and exploitative. If the company wants to incentive good quality of service, they can do so explicitly with direct ratings instead of some unclear way of tips.