Back court violation? Test question

[BamaRef]

I'm still not sure what the correct answer is and why. If the player had not passed the ball, but instead, had landed in backcourt then the play would have been legal the exception being that the player catching the ball on a throw in can land either in backcourt or frontcourt or land straddling the division line. Is the fact that he passed the ball to a teammate who is in backcourt the reason it is a violation as Matt S. states or is there another reason.