My feeling is that the non-US tourists understand exactly how to tip appropriately. It's the US-tourists who have lost their bearings.

Tipping is for extraordinary service. No tipping is required for standard service. The rest of the world understands this perfectly.

The strange US exception for waiters and bartenders (where, owing to a sub-minimum wage exception, they receive commonly a 15% "tip") has morphed in the minds of some US tourists into a de facto rule for other services. Only with truly extraordinary, above-and-beyond service should the traveler consider a gratuity.