I think we can't control the way the job listings are worded because they are usually a representation of the bosses' and human resources' preference in candidates. They can put any restrictions they like as long as it is legal, saying Drug Free Workplace doesn't guarantee it in the first place.
A company that is based in a state that has certain legal drug laws is working against itself if it shows prejudice against a legal user of those drugs. While the tech companies most certainly have a cadre of lawyers to fight off any lawsuits caused by this prejudice, it will take money from the profit oriented stock-holders, so it might just be prudent for them to erase three words.
If a company posting a tech position wishes to hire a candidate that is drug-free, that company has every right to state "drug free workplace" in the job listing. This way, if a drug test is a requirement for employment, the candidate already knows what is expected from them, and the company can potentially save time and money on what would otherwise be failed drug tests.
Like any other type of company, tech companies should make it clear to people what kind of workplace to expect. If someone smokes medical marijuana they would then know that they should bring it up in an interview to make sure they don't lose their job later because they are caught.