Like I said, I could have easily bought 2 litre bottles of water anytime, but really tried to keep my contribution of empty plastic bottles down (but did buy cold drinks at stores so can't claim nothing).
FWIW, I don't recall any time where I might have considered buying bottled water that they didn't let me use their tap water. In most (nearly all) cases they let me use ice from the soda fountain. I always bought something, asked politely, and thanked them.