Because it isn't safe to drive under the influence of marijuana, or to do many jobs. For driving, there just currently isn't a great solution, and that freaks a lot of otherwise open-minded people out. For jobs, the solution is often to disqualify anyone who has used it in the last few weeks (or even months, maybe?). Neither is a good solution.

Do you believe that breathalyzers result in expensive arrests and trials "for no good reason"? I don't. I don't believe people high on marijuana are as dangerous as drunk people, but I also don't believe they're harmless.