This unfortunately happens at every school. My nephew is a walk-on for a prominent Big 10 basketball program. As one of the few players who does not smoke weed, he is always "randomly" tested when the team is required to do "random" drug testing. In fact, it's a poorly kept joke on his team. He also has very good grades, which helps the team's overall GPA.
I suspect even UM knows who to "randomly" test and who not to test. The difference, however, is when the NCAA chooses who to test. For example, a few years back, the NCAA chose to test Michigan basketball player Mitch McGary, who was injured and not planning to play in the NCAA Tournament. But because it was the NCAA tourney, the NCAA chose who to test, and not Michigan. And lo and behold, McGary tested positive. The test of the Clemson players was conducted not by Clemson but by the NCAA. Which is why a player like Lawrence was caught. That would never have happened during the regular season when the school decides who will be "randomly" tested.