[Margaret Pilkington]

It would not be in the interests of an employer to test for sexually transmitted diseases unless that employer ran a brothel.....which is illegal in this country.

You ask what is problematic about these drugs, but this has been answered in previous posts...even you have put forward very good reasons why being under the influence of drugs or alcohol is unsafe.

You are just trying to fudge the issue.
What happens in America has little relevance to this issue......and tax evasion, marital infidelity....if they meant that you were not doing your job properly then an employer might be concerned.......and any issue that meant you were compromised in your job should and would be brought up at yearly appraisals, along with some actions to remedy any performance deficits.