I'm sure he's not the only one using things they shouldn't be but there are things on their Wellness Policy which are off limits which CAN be prescribed by the talent's own doctor. Look at Adam Rose and his ADHD medication. It really could have been something very small that was overlooked by both Reigns and the WWE and it only came to light when he failed the test or as you say it could be an excuse to slow down his push. If the WWE was firing on all cylinders and doing their best to create stars I could put a lot of weight behind that theory myself but who else is there? Who else have they even tried to make an effort with apart from Dean Ambrose and even then they're not really doing much with him. There's literally no one else that could tick the kind of boxes Roman Reigns could.
As far as people wanting to boo him, i'm not saying they can't boo him but don't boo him like he's letting the team down or he's putting someone else at risk or he's affecting someone else. The only person he's cheated is himself. It doesn't matter how hard Roman Reigns works, hard work doesn't determine where you go in the WWE, one man decides that, that's Vince McMahon, if he doesn't believe in you then it doesn't matter how hard you work or how early you arrive at the arena or how good you are in the ring and I think most of the people that boo Roman Reigns would boo him regardless, they don't care if he failed or passed any tests, they just don't like him and that's that.