Why, if they are not regulated by the dot, regulate them?
Liability. The DOT regulates vehicles based on size and weight, to minimize catastrophic accidents, where most of them result in multiple deaths or devastating injuries. Carriers regulate cargo vans strictly for minimizing their liability, to keep insurance costs down, and to keep their safety rating high.
What is the difference between a expedite cargo van running for 36 hours straight versus a U-haul running the same amount of hours? Or a motorhome?
The difference is who has the deeper pockets, the U-Haul or motorhome driver, or the expedite cargo van's carrier? There are other differences as well.
I guess the limit of time driven can be a benefit for the drivers who peter out after 250 miles.
That's true. On the other hand, drivers who say they drive 18, 20, 24 hours straight, and are just as alert at the end of the trip as they were at the beginning, are just flat out lying, 'cause they're not, and they know it. OK, granted, they might not actually be lying, they might instead just be stoopid.