How would it matter if there's a delivery of a product that's traceable? The claims in this case aren't that TradeHill didn't do what they said they were going to do, the claim is that the original transfer is unauthorized. The situation would be precisely the same if TradeHill had shipped them a physical product.
It matters because Tradehill were offering a service that had the equivalent of a big neon sign on it reading "ideal for laundering stolen money". Of course, they should've reacted by refusing to do business with Bitcoin marketplaces from the start.
Excellent point. I don't think Dwolla could have refused to do business with Bitcoin marketplaces from the start. Part of their business model and sales pitch is that they don't care what you do so long as it's legal.