Since Trey is gone Matt has played Zach that way some last year and almost every game this year. He played AJ that way against certain teams...clown pants being one. I think it is two folded...to help protect the rim, but mostly to try to keep him out of foul trouble. Last year Matt tried to keep him out of foul trouble by doubling the post and trying to get their 5 man to get rid of the ball so Zach wouldn't get fouls. This year Matt is trying to not get into rotations due to doubling, but still trying to keep Zach out of foul trouble by playing "drop" or one man zone in the lane. Zach is pretty passive until the game is winding down with fewer minutes and then knowing he has some fouls to spare gets more active on D which creates a run for Purdue many times.
Prior to that we keep thinking the drop is killing Purdue. It is consistent in Matt's overall desire to protect the rim and defend behind the arc while knowing how valuable Zach is on offense hopeful he doesn't get in foul trouble. By the same token, we too are happy when Braden shoots a midrange and makes it against the same defense.