I'm pretty much against it too for most jobs. I do feel, however, that there are jobs where it should be crucial to ensure that the person isn't on drugs. Even marijuana.. Ever been on a pot binge and still felt the effects the next day? I would hate to think that someone who is suffering from brain fog might be flying my airplane or administiring IV meds before a surgery or handling a large bottle of caustic acid. I'm pro marijuana, even, but I can recognize that it can be dangerous in certain jobs.
So yeah.. it might be an invasion of privacy, but for **certain** jobs, it's a must.