Shit happens; signs are confusing, drivers make mistakes, cops make mistakes. And that may be a once in a lifetime event for a regular driver, but when you have thousands of drivers parking dozens of times a day, "once in a lifetime" becomes "once a week".
Except very special and rare situations, the driver should be personally liable for a ticket even if on the job. Traffic laws apply to individual drivers and not vehicle owners or contractors.
I have a friend who works as a driver (merchandising, not delivery) and he personally pays for his parking tickets. This is in the UK though, maybe things are different in the US.
