they will never bring the draft back because eliminating it also pretty much eliminated the anti-war movement, but if they did I wonder what they would do about people testing positive for weed

Weed won't just be a problem for a hypothetical draft, but could eventually become an obstacle to recruiting. The military changes when they need more people. They relaxed many of the restrictions on tattoos, as too many potential soldiers were being turned away for their ink. Recruiters used to turn people away if they had a history of marijuana use, but now you could probably walk into a recruiter's office stinking of weed and they'll sign you up as long as you're able to pass the piss test upon entering MEPS.