If you ask for a juice at one of the pool side bars they charge you $2 and something. (I order an orange juice at the adult pool and the bartender ask other guy if they had juices and he used the plastic bottle they use to prepare drinks)
The selection at Topsiders was Water, Fruit Punch or Ice Tea. I guess the mugs are only good for sodas.
Juices are free at the stateroom but remember you have to tip for that service.