Any kind of accident opens companies up for lawsuits and/or fines...especially from OSHA. Since it's not possible to tell WHEN potheads last smoked pot any accidents caused by potheads must be assumed to involve impairment because of pot smoking.
If a companies defense is "well, I didn't KNOW he was high" it doesn't matter. They allowed a stoned employee to work. They'll be sued by people hurt by their product or service and lose in heartbeat. They'll be fined out of existence by OSHA if it's a workplace accident. It's much easier and cheaper to just not hire or to fire them if they ever have pot in the their system.
“Any kind of accident opens companies up for lawsuits and/or fines...especially from OSHA.”
The airlines are just terrified by this.