Academic integrity means honesty and responsibility in scholarship. Professors have to obey rules of honest scholarship , and so do students. Academic assignments exist to advance this goal.
Grades exist to show how well the goal is attained.
Thus, all academic work and resulting grades are assumed to be the result of the student's own effort to learn. Academic work completed in any other fashion is pointless, and grades obtained from such work are fraudulent. Academic integrity means understanding and respecting these basic truths, without which no university can exist. It violates the assumptions at the heart of all academic work. It destroys the relationship of trust and respect that should exist between student and professor.