Blame New Jersey. A 1987 NJ state law required all food products sold there to display an expiration date of two years or less from the date of manufacture. Labeling, separating and shipping batches of expiration-dated water to the Garden State seemed a little inefficient to bottled water producers, so most of them simply started giving every bottle a two-year expiration date, no matter where it was going.
Now, the U.S. Food and Drug Administration has never established or suggested a limitation on the shelf life of bottled water as long as it's produced in accordance with regulations and the bottle remains properly sealed. Even Dirty Jerz caught on to this fact and amended the law. But the expiration date has been an industry norm for so long that many producers have just kept it on there.
Better With Age?
While "expired" unopened bottled water isn't going to do you any harm, it isn't going to get better with age, either. The plastic that water is packaged in -- usually polyethylene terephthalate (PET) for retail bottles and high-density polyethylene (HDPE) for water cooler jugs - is slightly porous, so the water can pick up smells and tastes from the outside world. Keep a case of bottled water in the basement for a year or so and it's going to pick up some interesting flavors. There's nothing better on a hot summer day than a 2007 Evian, with hints of dust and a crisp kitty litter finish!