My point is that potheads probably don't call in "sick" from a weekend bender hangover, but we know alcoholics do. We test for marijuana use, we don't test for alcohol use. We know for a fact that even a teetotalling, cautious person calls in real sick every once in a while. So the reason for drug testing being "to avoid absences" is logically inconsistent.

Its incomplete; still useful if we assert that potheads will miss work.

Stipulated. But experience with co-workers tells me that drinkers miss more than potheads work, and can have catastrophic life collapses. "Drug" tests don't often test for alcohol consumption. So, I still think that "drug" tests aren't in place for that particular reason.

Arguable. I personally don't think any invasive test is warranted. Measure my performance; all else is irrelevant.