This probably has to do with the money laundering rules and the "know your customer" regulations. If it were a cash transaction, it would have to be recorded if it met or appeared as though it could meet the daily limit.

I realize you're using a credit card to buy the gift cards, but back when the "use AMEX card to buy AMEX Travelers checks" churn was going strong, folks ran into this regularly, see, e.g., this post.