If you get in an incident and insurance is going to pay out, for sure they'll test you again immediately after the incident.

I had a job where I oversaw some people who drove company cars and if they were ever in an accident, they were sent immediately for drug and alcohol screening - like as soon as they'd leave the scene of the accident unless they were taken to the hospital.

I imagine the policies are written under the assumption that employees won't be allowed to drive under the influence. Policing that is on the employer, not the insurance company. And the insurance company demands an immediate drug/alcohol screening as proof the employee wasn't under any influence, or else they won't pay out.