I'm not sure what this hypothetical product would measure. Driving ability? If so, what if it was found that that would discriminate against classes of people? How do you gauge driving ability anyway? I would probably win a formula one race against my wife, but she's much better at paying attention to the road on long, boring stretches of highway. But I can't imagine that it's easy to test for that.

Meanwhile, this product tests for something that's easy and simple. Have you recently used marijuana? It gives a yes or no (and presumably a level of some sort). It's not the perfect test, but it seems like a fallacy to argue against this test on the basis of a better test that doesn't currently exist.