There’s nothing to indicate that drug testing is their only method of detection, it merely implies that those who have been found to be subpar also tend to fail drug tests.
What’s needed are tests that can differentiate between someone who smoked a bowl or two last weekend and someone who is impaired when they are at work.
I think it should be legal, I don’t care if someone does it; never had problems with a stoner- outside of a roomie eating a pie I made - drunks and tweakers and cocaine users are another matter entirely.
It’s also a state’s rights issue; I see nothing in my copy of the Constitution that gives the federal government any authority to regulate what is in my bloodstream; at least the prohibitionists got an amendment passed, as stupid as most people think it now.