"The apple juice"
grammatical properties of compound words are defined by its rightmost part/morpheme
There is no particular reason for it, I suppose. You may have to learn if the singular or plural is used for every kind of juice. Traube-n-saft - Traube = grape, Trauben = grapes, Mangosaft, Birne-n-saft - Birne = pear, Birnen = pears. Germans have similar confusions with different acids. Butyric acid = Buttersäure (Butter = butter, Säure = acid), malic acid = Äpfelsäure (Apfel = apple - Äpfel = apples(!)), formic acid = Ameisensäure (Ameise = ant - Ameisen = ants(!); or jams: Erdbeermarmelade (Erdbeer(e) = Strawberry, Marmelade = jam), Feigenmarmelade (Feigen = figs(!))
well, it is confusing sometimes and you have to learn by heart a lot but eventually you will get a feeling for it (and just stay wondering about Äpfelsäure like every german in every german chemistry course..)