Seriously???
They're testing for glucose in your urine, an indication of diabetes. Undiagnosed or untreated diabetics can pass out at the wheel of a vehicle, not a desirable condition for driver, company, or innocent bystanders.
Swings are CDL-Haz endorsement required, TSA screened. They are required for transporting DG in excess of limits.
Couriers aren't.
That's why swings are placed in their own pool of candidates for random drug tests. The smaller pool means being selected more often.