Of course, the authors, Jenny and Dave politely restrict this to Americans. But ABCDs, Amrikaa return, etc also join in.
the underlying foundation of this fear is actually far more fundamental:
(1) Drinking foreign water could lead to
(2) Getting sick in a foreign country, which might then force us to
(3) Use a foreign bathroom.
This is the sum of all American tourists fears: the receptacle that awaits if the uncontrollable urge hits far from the hotel lobby. Not unsanitary conditions, mind you (bus station bathrooms are the same all across the world), but the very concept of squat toilets, and the mere thought of a bathroom lacking toilet paper.