Right up there with packets of nuts containing the warning 'may contain nuts'
Septic Tank Emptying

Because toilet water in America is treated to the same level as tap water, it doubles as an emergency back up water source in cases of extreme power outages which would render central water undeliverable. (Lab tests have shown that there is less bacteria in a common toilet than there is in a kitchen sink). The reason why that sign was one the toilet is because it is using untreated rain and/or gray water, which does not meet drinking water standards. The signage might sound absurd to some people, but there is actually a reason for it--and has become a code requirement whenever rain or graywater is used.

WTE says: We also use potable water for flushing toilets in the UK, but we have never known anyone who would consider drinking out of one, even in an emergency! Let's see - drink from the kitchen sink or the toilet?? - Different strokes for different folks I suppose.

You could always put a loo block in and freshen breath at the same time.