In any case, the difference between a cake with arsenic and a cake without arsenic is going to be significantly less than 1%, but I'm going to make my decision based on that less-than-one-percent.
For health reasons. They're very rarely criticized for their environmental impact, when they're impact is almost certainly greater than bottled water (since they're not just water and a plastic bottle, but water, a plastic bottle, plus sweeteners and additives).
Some schools have banned bottled waters but allowed bottled soft drinks.