Someone was talking on CNN about legalized marijuana in Colorado. The problem is that even if it's legal to smoke it, the amount of time it stays in your system on a lot of variables. So what they were saying on CNN is that even though someone smokes it legally, companies don't like for employees to be under the influence of drugs or alcohol because the employee could make a mistake causing on-the-job injuries. That's a big liability for employers. So if you work in a "drug-free zone," your employer has the right to test you on the spot if you have an accident on the job. And bye-bye job.
It just means all the jobs will migrate to places like Colorado, Washington, and California. For years a lot of employers of a lot of high tech companies for instance have had a no test policy. If Silicon Valley tested for pot use, you could kiss your iphone goodbye.
Companies could forbid you from drinking alcohol if they wanted to, but they wouldn't have any employees if the implemented such a policy. It has to do with the widespread acceptance of the drug all over the country. As pot becomes legal and accepted in more and more places, employers implementing draconian test policies will no longer be in a position of power.