I see what you're saying, and of course it is possible that some jerk is buying gift cards with the card.
That said, it's worth pointing out that the tweet stream is not real time and it doesn't have every transaction. The card balance is scraped from the sbux site every minute and posted to Twitter if it has changed since the last tweet. When there are multiple transactions in the same minute, the changes get aggregated. This can result in some strange looking numbers.
What I think we're learning here is that if someone wants to reload the card, lots of little transactions over time is better than one big one. Or, perhaps the balance should not be tweeted.
Thoughts? j