Its all quite simple. Intelligent, hard working people that normally would succeed will do so with or without marijuana.
People are let go all the time because they're not intelligent enough, or because they're intelligent but lazy. Their decision to use marijuana doesn't change their values, they're just naturally dumb or a slacker. The only reason anyone knows they use is because they didn't care about hiding it.
From my experience in both tech and the business end of petroleum, most intelligent people realize this fact, which is why the majority of smart people don't care. There's a helluva lot more problems to deal with than some grass.