Water purification - necessary?

No - not necessary. Drinking fountains are usually marked "potable" or "no potable" - i.e. drinkable or not. Often there is a sign which makes it clear whether it is drinkable or not.

I've always assumed the water is safe unless it is marked "NON-potable."

In that case, what I understand is that it simply has not been chemically treated.
Last year Joe drank profusely from a fountain noted "non-potable" and he did not get sick at all.
That could have to do with the shots of straight whiskey he drank about 15 minutes after discovering his error... or not.

But no, you do not need to treat the water.
It is excellent.