In my experience, Starbucks is one of the few places which allows transfer of gift card balances or purchases of gift cards with other gift cards.

I can only assume this is because gift cards are intended to behave like cash (when the balance is zero, they have no additional value), but operate under the mechanics of credit/debit cards (asynchronous transactions, network failures/delays, etc.). Debit cards have the feature of going negative with very clear legal liability assigned to the account holder. Gift cards holders are essentially anonymous, so recourse may be more difficult.