Based on the premises that all drugs will show up on a test, and that use of (illegal) drugs necessarily impairs the job performance
of any employee
The first premise is known to be false. Many drugs are completely out of your system in less than 24 hours, including LSD and ecstasy. Use of these drugs will show up on a hair test, but not in a blood test or a urine test, and most companies test urine.
The second premise is also false. It is related to the war on drugs. It is based less on which drugs actually mess you up the most: while your employer is testing you for drugs, coffee, tea, pop, and various sweet things are available to you for little or no money.
Drug testing also indicates to employees that their skills are less important than whether they pass the test or not, thus demeaning their skills.
The idea that unpaid hours should be under your employer's control is a step away from an employer mandating amphetamine use on the job to improve performance.