PDB (pre departure beverage) is basically that - a complimentary offering under limited time constraints. Sure, they can get something else but that would take a bit more time and personally, I think I would want to start the passenger - cabin crew relationship off on a good footing
I understand that there may not have been time constraint s on your flight, but I see PDB as a simple gesture, and getting something else requires opening the drinks trolley and glass trolley which the cabin crew are required to stow away. ...
While I don’t think I have any issues with a consistent offering based on two choices - this is a bit of a stretch to try to make out like pouring a glass of sparkling water is some time consuming burden, as if the exisiting water, OJ and glasses don’t come from and go back into the exact same trolly.