The reason water towers do not freeze is because the water tower is constantly draining and refilling through the day.
The water in the water tower does not just sit there and not move and so it does not freeze just like when you leave your water faucet running it helps to keep the water from freezing.
Moving water is harder to freeze than water that is not moving.
The air up above where the water tower is is also not as cold as the air down on the ground.
So it's warmer above where the water tower is.
I've heard of a water tower freezing before in some town but it's rare that it happens.