I think they have moved away from cargo ratings. I haven't seen it listed last few times we bought. While you could back calculate it, it's more limiting.
Example a family of 4 with a 4 door 6 seat pickup. Assuming a couple of 50lb kids a 200lb husband and 150lb wife, that's 450lb of payload used vs 900lb used by your cargo assumption. You just lost 450lb of usable payload to the assumption working off your cargo rating calculation.
Really unless you are trying to push the truck to the limits, it doesn't make much difference and if you are pushing to the limits, false passenger weights you will never see don't make sense if you need that last 450lb of payload.
If you read the sticker on the door, it's quite clear what is included in payload.