The fact that you sold if for an amount agreed upon by you and the buyer is what matters, not any provable intrinsic value the thing being sold may have. Whether you sell $1M diamond or a 5-cent bottle cap for $100, that's $100 in damages if the processor does a fraudulent chargeback.
If you substitute "TradeHill" for "E*Trade" and "bitcoins" for "stocks" the problem would still be the same, right?
This is in contrast to almost any other exchange system (besides hard cash), which could, at least technically, force reversal of a transaction. See for example Paypal, Facebook credits, money in an MMORPG, shares of stock, and so forth.
With bitcoin if I hold my coins in a currency exchange (which is basically a bank) they can obey the court order and reverse transactions.
And I can become my own bank, and then hide, in which case it would be hard to serve a court order on me. (Although banks often have accounts at the fed, so they don't hold the money themself.)
What's unique about bitcoin is most people hold their own accounts. So you have to find the account holder, which is hard. But, as I said, it's not the exchange the matters, it's the account holder.
It would be trivial if it was that simple, but the stolen funds were of course withdrawn from Tradehill, and so are no longer in control of an exchange.
It's no different from a criminal running off with a bag of cash; how are you going to "reverse" the transaction of the money being physically picked up, put in a bag, and carted off?