The rule is ridiculous. If you are caught tested during an NCAA tourney game, or I think a bowl game, it's a one-year suspension, first time. If you're caught outside that (because your school tests, for instance), it is up to your school. If McGary had been caught in January (and reportedly he was tested 8 times by the school in his first two years), he'd have been suspended two weeks under Michigan's (which is relatively harsh by most school's standards). But because he got caught two months later, when he wasn't actually playing, mind you, he's suspended a year.
He'll be fine in the NBA, where it takes three positives to get suspended for 5 games.
That said, my guess is he's not fully healthy for draft preparation, because he would have returned to school but for the suspension. He very well could slide to the second round -- if he did, it's tough for me to imagine a second-rounder I'd want more. Hopefully he can land on his feet in Boston.