Actually most "responsible" jobs won't drug test because it becomes too difficult to find people who are experienced enough to take on that responsibility. If they did they wouldn't be able to staff a shift. As long as they're well enough to do their work everyone will look the other way.

On the other hand, jobs where the employee is easily replaceable will be more likely to be drug tested. There are people literally begging for the chance at a paycheck. It's not really about safety or morals and more about employers maintaining authority over employees.