For a scammer tag, the accused person needs to have promised to do something and then failed to deliver on the promise. TradeFortress never promised to pay anyone any bitcoins here. If you trust him to do something that he didn't promise, that's your problem.
Just facts: TradeFortress issued IOUs and sent them to users who trusted him. IOU means "I owe you". Thus he admitted that he owes to that people.
Question: If one owes to people who trust him, but is not going to pay them, then who is that person?