Because bottled water doesn't have real or imagined therapeutic properties, and lithium does. Eventually someone unfamiliar with the rules will advertise a lithium/water product as having medical effects, and that will end the game (advertising medical benefits is a no-no without both research backing and FDA approval).
EDIT: this is another example of the "fact rule" -- if you post something factual and uncontroversial, you will be downvoted with a probability of 1.
I disagree. There is a market for supplements, as long as they walk a careful line of avoiding the promise of any medical benefit. Lithium could be sold this way, in low dosages ("Lithia Springs water"), but it's probably not allowed? I remember that 7-up once contained lithium, but it was banned, even though the amount was very small.
BTW: There is indeed an annoying level of unnecessary downvoting going on.
EDIT: this is another example of the "fact rule" -- if you post something factual and uncontroversial, you will be downvoted with a probability of 1.