I think they're assuming a sane defense strategy that doesn't involve Nick or Kayla taking the stand and making wild allegations that the drugs belonged solely to April or Aaron or someone else. If they do that, as dumb as it would be, and the state leaves that unrebutted, it opens the door to a closing argument that is basically the only people who know for sure whose drugs those were were Nick, Kayla, April, and maybe Aaron. The only two who testified were Nick and Kayla. The state could have called both of the others. They didn't. Why not?
I think this would be idiotic but Nick is an idiot.