Confused! Auto parts stores sell deionized water for auto use only; claiming they are prepared solely for this use. Not for human consumption. What do they do differently after making DI water? Are there special additives like in gasoline? I understand there are four different types of DI water.
Probably a couple reasons.
For something to be mass manufactured and marked for human consumption, it needs to go through a certification process, needs to be stored in food-grade containers, etc. The deionized water didn't go through this process, and probably isn't in a food-grade container.
Also, it may contain trace chemicals from the deionization process which wouldn't be healthful. Because the goal of the water is not for humans to drink it, the manufactures just put a "not for human consumption" label on it and sold it without going through the effort to remove these chemicals.
So while there aren't any specific additives which you should be concerned about, the manufactures don't guarantee that there won't be any trace chemicals, and recommends against consuming it.
(Motor oil doesn't have any warning, but you shouldn't drink that either. Antifreeze coolant has a lot of warnings because it'll poison you. Deionized water is just a less obvious non-consumable than motor oil, and the water is less poisonous than coolant, so a less severe warning was applied.)