Same as it happens when people wash their veggies at home, or use their dishes they've washed with the tap. Nobody worries about consuming small amounts. You know, it might not even be a big deal to drink from it, it's just not done. It's not that it's guaranteed to be undrinkable, it's just not guaranteed to be drinkable, and there's a difference between consuming a few milliliters to consuming a few liters.