Let's Duel

Why is the battle between Yvain and Calogrenant viewed as dishonorable (or like a crime) when it was fought fairly? Everyone in support of Calogrenant viewed Yvain as a terrible person even though they are the ones who set up traps in their castle, and they send a knight to go fight whoever causes the great storm. The people act as if Yvain hurt Calogrenant's horse or took a cheap shot at him (De Troyes, 267). Sir Yvain has a point when he states "when your lord attacked me, was I wrong to defend myself?" (De Troyes, 281).