The only reason I can think of is that water absorbs into the body once it has reached your body temperature. The colder the water...the longer it takes to absorb into your body...thus taking longer for your body to rehydrate.
If one is close to heat exhaustion, when arms muscles start to cramp up, it's not a good idea to drink whole glasses of ice water at once. Give the body time to return to normal. If you are just hot from working or exercising, drinking ice water is OK, if not a quart of it at once.
We are living in India where temp rises from 40'C to 49'C with dry atmosphere having humidity around 15 to 35 % , If we drink fridge water in such a hot season we may get stroke, or sudden fall in body temperature can cause some problems so just to avoide we should drink just cold water or mixed fridge water.