It is policy not rudeness.
They give free ice water to keep people from dropping like flies. In fact the 'rules' are to simply walk to the counter and ask for a cup and they either have them setup already to hand out or they just fill one up really quick. No need to wait in the line to order it at a register. Heck multiple times when I was really thirsty I've asked at table services and been given a to go cup of ice water without as much as a sigh or eye roll.
Disney hands out water for free in hot lines as well so I guess they didn't get the memo to only hand it out to those holding park bought food.