No reason. In MAD I was allowed to drink the water, in ZRH I wasn't even allowed to touch the bottle in case I activated the secret combustion function, and in FRA the screener claimed he was not allowed to touch the bottle in case he activated the secret combustion function. At LHR they didn't see the bottle on the x-ray, and I didn't notice until it was discovered before my next flight. And you can take as many liquids as you like on domestic flights in certain countries.