I admit, I had never thought about it either but it would be entirely possible for the babies to drink too much and die. Too much water too quickly can make any animal sick. Humans can even die of water toxicity. I do agree with you that these particular instructions should be BOLD! There is no reason that you should have to search the fine print to find this out.
Baby poultry can survive up to 3 days without food or water because of their yolk that is absorbed right before they hatch. It should provide the necessary nutrition and moisture for them for that amount of time.
The reason for this is because the mother often stays on the nest for a few days waiting for all the chicks to hatch. There wouldn't be very many babies survive a hatch if the first ones couldn't wait for the last ones. The mom would have to choose to either get off the nest and abandon the unhatched babies, or let the first ones die of thirst and starvation while she waited for the others to hatch.
Shipping is hard on them and is stressful. They arrive hungry and thirsty! However, if the carrier actually keeps them from extreme temperatures and handles them as quickly as possible to ensure the shortest transit time as possible, the death toll should be minimal. Now, whether the carriers actually do that or not is up for debate. They are supposed to. But, they are supposed to handle "fragile" packages with care, too, and we all know that doesn't work either.
DeAnna