Why is there an expiration date on bottle water? Is it drinkable after that date?

http://www.bottledwater.org/public/faqs.htm#11
FDA has not established a shelf life for bottled water. IBWA advises
consumers to store bottled water at room temperature (or cooler), out
of direct sunlight and away from solvents and chemicals such as
gasoline, paint thinners and dry cleaning chemicals. Bottled water can
be used indefinitely if stored properly.

Perhaps apropos of nothing, there is an urban legend out there that
plastic bottles can eventually degrade, leaching harmful chemicals
into the water it contains. This notion is reasonably considered to be
bunk (see http://www.snopes.com/toxins/bottles.asp for example) but
it's an idea that is out there...