It is important for employers to be allowed to do this so that they can get the best possible work out of their employees and so they can have less to worry about in terms of workplace accidents.
If an employee is using drugs, they may well be impaired at times when they come to work. This will lead to them being less efficient in their work than they should be. In addition, depending on the sort of job they do, there comes to be an increased risk of accidents. These accidents could harm the impaired employee, other employees, or both. In other words, employees who use drugs could well cause harm to their employers in one way or another. Therefore, it is important that employers be allowed to conduct drug testing.