Free Library?

Can anyone tell me why some it says "Free Public Library"? I always thought all Public Libraries were free... Hence the word "public".

Seattle Library

This is a picture from Seattle's Rem Koolhaas Public Library. It is a modern art-type-building. It's actually really nice... This though, is one particular "art" that I didn't really understand. It's -I think- holograms of faces that are reciting -once again, I think- parts of novels. It's located beside one of the escalators. It scared me quite a bit... What do you guys think?