You do have to pay for Rootbeer / Dr. Pepper - even in the dining rooms. For some reason there is no Dr.Pepper or rootbeer available free of charge. We were told by our servers that it is available at a cost.
You do need to pay for all Soft Drinks ordered through room services as these are in cans.
The free drinks at the pool deck have small cups - so I would recommend that you bring a water bottle or larger cup or mug and fill it up and bring it back to your room to have with your room service.