In Hebrew it is 'drink/juice of the fruits' plural, not singular. eg. מץ תפוזים
That's true for most juices, but not for all - for example: carrot juice = מיץ גזר and lemon juice = מיץ לימון. I don't know why some fruits are plural and others singular, I don't think there is a rule behind it unfortunately.
The noun משקה doesn't change in construct state. Notice that משקה is not a feminine noun, and that it doesn't end with "-ah" but with "-eh", so it doesn't behave like those nouns.