I would guess that since you are going to be operating heavy machinery, or be around it, IE forklifts, insurance dictates that you need to be drug
free. i have seen alot of my jobs drugtest because of this, they just need to cover their asses. Imagine if you are working and some high as hell dude cripples you with a forklift. the company wants to be able to say they took all the required precautions to prevent drug users from working there. That amazon is not held liable for it. This is all speculation, just what i figure.