As you said, they're intelligent.
They know that it's a cage, even if it's a gilded one, and depending on the drug, they've opted to free their mind.
Drug tests are to flag drugs they don't want you to use. Alcohol and other drugs? Fine. Carry on.
It's a test to see if you obey the law.
If you feel free to break the law, you will feel free to break other rules.
People that do drugs have higher on the job injury rates, high use of sick time, show up to work late.
Before you say it, alcoholics do as well, but not everyone that uses alcohol is an alcoholic.