I happened to email Radley Balko the story about Kessler's moronic gift card proposal, and he immediately posted it on Reason's Blog, where an interesting discussion on the topic has started.  One comment in particular was very insightful, and gives the most concise and clear explanation I've seen on why stores put expiration dates on their gift cards:

For retailers, gift cards are both a financing blessing and an accounting curse.

On the blessing side, they get to collect cash up front (yes, cash is still king), earn interest on the float before the card is redeemed, and profit from the percentage of card dollar-value which goes eternally unclaimed. Basically, it's an interest free loan that the lender sometimes forgives.

On the curse side, that profit from unclaimed cards never makes its way to the bottom line, because it remains an outstanding liability (unearned revenue) that offsets the asset value (cash). Without an expiration date, the unearned revenue account would, in theory, never be settled. To deal with this, firms have to keep track of every card sold and its value, so that they can account for which of the liabilities have gone stale (according to their own definitions), and write them down at the appropriate time. Then for every person who happens to find and use a card that was previously written off, the firm has to do a reversal on the value and reduce their profit.

From the company's perspective, it's much easier to stamp an expiry date on the card and transfer the balance from unearned revenue to the revenue account as soon as the expiry date arrives, so they can book the profit and be done with it.