Well, you are right to some degree. I was a little too specific when I stated warranty work only. I meant more warranty than safety related, my mistake.
Obviously the manufacturer wants to keep warranty repair costs down. Having lower payload and tow ratings helps keep them from fixing a lot under warranty.
Manufacturers also want a good reputation and want people to continue buying new vehicles. Keeping those ratings below the true actual rating helps with longevity... hence why manufacturers derate engines.
Manufacturers want to reduce liability. If they know the failure point is 100 lbs, they want a nice compromise between longevity, ego and failure. If that means they rate that piece at 60 lbs, they know they are way below the failure rate and it would take an obvious excess of 60 lbs to cause failure. They want to be able to prove that they rated it so far below the failure rate that it couldn't possibly have failed due to the manufacturers negligence.
I misstated my point by saying WARRANTY ONLY. Either way... its not a SAFETY ISSUE.