The catch is that the interest-free loan can be called in by you, the gift card holder, at any time - so they get less utility from any given amount of gift card balances than they would loans/corporate bonds of the same amount because they're always trying to be prepared to pay out some portion of those balances.

This is why they're treated as a liability in the company books. You can guess or bet that all of your outstanding gift card balances won't be redeemed at once, but there's really nothing preventing that from happening and causing cashflow problems for the company. And there's lots of overhead involved in tracking many many thousands of small balances on cards into perpetuity.

Much easier to encourage people to spend the gift cards and get your financing from proper, predictable business loans or bonds.