IOW, they can yell at them if they are looking sleepy and the like. But the drivers fear more sinister reasons ranging from they can catch them picking their nose or listening to rightwing talk radio to "inappropriate conversations" on a hands free device.
I would object to that, too — but I understand completely why companies in the trucking industry do that. It’s their way of covering their asses in the event of a lawsuit … and I’m sure UPS gets sued with such boring regularity that they keep an entire law firm busy every day of the week.
Amazon trucks, many of which are contracted to small companies that operate them in fleets, are loaded with cameras and sensors and the like. They use their Alexa tech to track everything - and know where the drivers.vans are, can see in all directions, they know if the seatbelt is fastened, they know if the driver is speeding or ran a stop sign.
There is an Amazon fulfillment center up the road. I have a vendor and a client nearby and If I drive up there in the morning I sometimes get stuck at a stop sign and watch literally dozens of those Amazon trucks heading out for their daily deliveries. I also know where the drivers get there gas - best to avoid that station at the end of the working day.