The "no drinking water" signs have to do with health requirements for drinking water. Water for firefighting, for example, has to be isolated from drinking wat

The "no drinking water" signs have to do with health requirements for drinking water. Water for firefighting, for example, has to be isolated from drinking water to prevent any risk of contamination. Due to how it is used, this water may have to be stored for very long periods of time and provides a breeding ground for all kinds of things you wouldn't want in your drinking water (e.g. legionella). I'm fairly certain most (non-drinking) fountains use something like a semi-closed loop system where the same water just circulates through the fountain and fresh water is only used to replace any water lost to evaporation or spilling. This means the water is easily contaminated and therefore not suitable for drinking. In some public restrooms (though this seems very rare), even the tap water is labelled as not safe for drinking. In these cases the tap water is likely not sufficiently isolated from the water tank for firefighting and therefore not safe despite coming from drinking water.