The sender has two options with promised coins, finish payment, or destroy. Destroyed coins are returned to the unmined coin count or just eliminated from circulation.
This essentially eliminates the receiver's benefit of defrauding senders. Senders also cannot defraud receivers since they cannot get the coins back, they can only destroy them.
This doesn't really work. What typically happens in the fraudster says "Sorry, I can't go through with the deal because
. Release the funds and I'll reimburse you." Now, what's your incentive to destroy them?
Bingo. Denial of service.
Suppose NSA/SS/Random asshole/bitter ex just want you to lose money. They can start a transaction, and after you send the promise, ignore you thereafter. It cost them 0 bitcoins, 1 minute of typing, and gives them 10 minutes of trolling entertainment. It will cost you the entire transaction amount. Classholes will even write bots to do this for the lulz, I promise.
Please excuse the cliché hollywood quote, but: "Some men just want to watch the world burn.".