"Tipping confounds me because it is not a reward but a travel tax, one of many, one of the more insulting. No one is spared. It does not matter that you are paying thousands to stay in the presidential suite in the best hotel: the uniformed man seeing you to the elevator, inquiring about your trip, giving you a weather report, and carrying your bags to the suite expects money for this unasked-for attention. Out front, the doorman, gasconading in gold braid, wants a tip for snatching open a cab door. The bartender wants a proportion of your bill, so does the waiter, and chambermaids sometimes leave unambiguous messages with an accompanying envelope, demanding cash. It is bad enough that people expect something extra for just doing their jobs; it is a more dismal thought that every smile has a price."
That's Paul Theroux in Dark Star Safari: overland from Cairo to Cape Town. He's on a cruise on the Nile, looking at ruins.