Yes, a "drug free workplace" should be posted in a job, this allows for the openness of information to applicants. We live in a state where you are permitted to purchase marijuana, but not all of us care to do that kind of drug. I hate the smell and that would bother me during work if I smelled that all day long. Some offices do permit that on an employees own time or break, but they need to be aware. It should absolutely be posted!
Whether or not one chooses to use drugs recreationally is irrelevant, an employer has the right to say just about whatever they please in a job posting. Employers should have the ability to screen for drug use if they want to, and stating that it is not tolerated in the ad just saves time.
Any job listing should include as much information as possible about the job. This helps both employers and job-seekers. Including the words "drug free workplace" in the listing will deter job seekers that cannot pass a drug test and save time for the employer since they will not have to deal with interviewing those who do not qualify on this count, It will also save time for job seekers who cannot pass the drug test, even though they may be qualified otherwise.
Rather than saying "drug free workplace" on the job posting, businesses that actually want to have a drug free workplace should instead simply have a little warning on job posts that state all applicants must pass a drug test. That would discourage any drug abusers from even wanting to apply.