Not to mention, if you're buying Water, that means you are buying Water exported from another location (think about exporting rivers, lakes, aquifers.) Once the water makes it to a store, it can no longer ever be put back in the ground it came from. At least with waste water in your drains, it has a chance of making it back into some aquifer or river. Maybe a small one, but better chances than that shipped to a store.
PLUS now you're increasing the carboon foot print by supporting the use of gas / oil to transport the water from a production facility to a ware house, from a warehouse to a distribution center, from a distribution center to a local retailer. From a local retailer to your house.
The amount of gas/ oil used to transport the water, plus the amount of water used to produce the gas / oil, plus all the emissions one is supporting.... Not only are you paying a premium, but, now one is supporting the carbon foot print of water exports. All to avoid the production of your own wastewater? Which in the end the production company is going to have as much waste water as you. So essentially, this doesn't accomplish anything, but increase the greenhouse emissions and increase oil / gas usage.
Which we all do, I guess if it ends the arguement...