Are you completely sure that it was someone you didn't have a deal with?
Even if the deal doesn't go through, but money was entered into escrow and there was an initial agreement to make a deal happen, then the counterparty does have the right to leave negative trust. Otherwise, if facing a seller with ill intentions, does the buyer have to complete the trade just to leave feedback? It is probably designed to be this way.
However, I do see an option of blocking traders without even opening a trade with them in the first place, which is quite odd. Though, I don't think that has anything to do with your trust score.
Trader doesn't risk any money on it because I am selling my bitcoin to him.