I think it's because generally "nuoc" (water) doesn't imply its for drinking. I live in Vietnam right now and every time I say nuoc they don't understand so I must say La Vie (common brand) or nuoc tuoi, which I believe means fresh water.
I think regular nuoc implies industrially used water or tap water, which isn't drinkable here. Can any Vietnamese confirm this?
"Nước" can also mean "country", in example "nước uống" would be water to drink, but "nước ngoài" would mean something like "outside country" or simply abroad. Since I'm not living in Vietnam, I can only guess that they expect you following up with "a second part" of the word or they don't correctly know what you want.
Good point! Perhaps in Vietnamese, it is used as adjective or just a compound noun. ;)