What I don't understand is how a trucking company that has their unit jammed doesn't eventually figure that out and fire the driver.
[EDIT] The only thing I can think of is that the GPS is always recording, but only examined in the event of an accident. The driver can claim the unit was defective and not his fault, so there's no proof he was driving 90 in a 60 zone.
It's possible that early units weren't very reliable. If drivers started using jammers before the reliability improved than the company may just expect a high rate of failure.
The other option is that insurance companies and regulators want the GPS trackers installed, but the companies don't care as long as the deliveries get made.
There are limits on how many hours a driver is allowed to work, for safety reasons.
[EDIT] The only thing I can think of is that the GPS is always recording, but only examined in the event of an accident. The driver can claim the unit was defective and not his fault, so there's no proof he was driving 90 in a 60 zone.