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I don't believe you can, since Starbucks lets you use your card for any purchase. That said, even if you could, why restrict it? He mentions that the point of this is to be "an experiment in social sharing of physical goods", so making it something purely built on trust makes the experiment exciting and worthy. If someone does abuse it, then that's just a data point in the experiment; but, if it continues to be replenished, that's a neat observation and a certain +1 for human nature.

Looks like a bad apple or two has helped take the over $50 that was on it an hour or so ago down to $0. I don't see a normal pattern of withdrawing small amounts of money for coffee, that's for sure.

It's possible someone could abuse it. It will be interesting experimental data to see if this happens, how often it happens, and how much it happens.