If I have a few drinks and go to bed- I hurt no one but my own liver.

If instead I get in a car and drive I could potentially hurt one or more people as my reflexes, concentration and judgement are all reduced.

This is what the device will do for drug users. Identify those that have more than a "safe" level in their system for driving a motor vehicle.

That sounds reasonable to me as does your argument that the drug itself has not killed others -it is the user that may...