It's a surprise to me too.  I think there has been mention of this in the past year on the towing forum.  Reese just says "cargo" weight, and no mention of what's past the rear axle.  Depending on your truck, you could add anywhere from near zero on a short trip with just you to around 1K lbs if you add DW, kids, dog(s), firewood, generator & fuel, bottled water, etc.  So you should have two sets of bars depending on what you've got loaded?  
 If you have bars that are capable of adequately transferring the correct amount of weight back onto the steer axle, why would you need stiffer bars?  And in the past few years, truck manufacturers have reduced the amount of wt. to be transferred.  Ford for ex. now says 50 percent of the wt. is to be restored back onto the steer axle.  Not only doesn't seem right in itself, I have not heard of any other WDH manufacturer stating this. 
Airstreamer67 wrote:
Quote "...I *think* Reese did not include the cargo weight in the past and this is a more recent change.
 "As an example, say your truck has 400 lbs of passengers, groceries and camping gear and you had 800 lbs of TW fully loaded, the spring bar should be rated at 1200 lbs. If your cargo was 600 lbs, you'd need their 1500 or 1700 bars depending on if you had round or trunnion style bars. Or another way of looking at it is if you had 1200 lb bars and 800 lbs TW, you'd limit your cargo weight to 400 lbs. I suspect that Reese owners would tend to undersize their bars without referencing the chart."
This is a surprise to me and goes against the conventional wisdom I've read before. What happens if, like me, someone loads 90 gallons of diesel in an auxiliary tank in the bed plus other junk to just about max-out the GVW rating when the trailer hitch weight is included. Is Reese now saying the poor weight transfer bars and the trailer frame are supposed to support all that? 
Hard to believe.