The Queen wanted to save the knight, she "...and other ladies so Prayed the King for so long for his grace That he his life granted him in its place..." She wanted to have knight so she could decide his punishment and not the king. "The Queen thanked the King with all her might;"
So the king granted her wish to decide what to do with the knight. This foreshadows how much the king pays attention to the queen because it is surprising to see how the king agrees with the queen and it is not something common that occurs but in this case it did.