I would guess they don't cater out of SLC, not uncommon to not cater at a non hub. In those cases they stock the plane for departure and return. I had a not to pleasant return from SLC once where the plane was somehow disconnected from electric overnight and we were told that everything including soda and water were all frozen, and there would be no service. They did tell us before we boarded at least.
Still, if I worked for UA, I would have served tomorrows water and went and bought a few water bottles at the airport and tried to expense it in the name of customer service, but the way United runs things now, that would probably get a co-worker fired.