I'm guessing there's some legalese in the contract between the employer and the truck driver they hired about using the employer's vehicle for "personal use". I'm guessing some of the drivers who used a truck they did not own - may be out of a job at a minimum.
I'm sure the employer has some way to penalize the drivers if they used their employers truck.
At the very least - as an employer in the near future if the truckers demand an increase in any sort of benefits - as an employer I would "cry" about how much the truckers actions effected the employer's bottom line - and that there is little to no money after paying the expenses.
If the truck driver owned their truck - that's a different story. not sure how that would play out.