In terms of payload rating the big difference between the F350 and F450 is the F350 can come with a gas engine which weighs a lot less than the required diesel in the F450. This gives the F350 the big payload rating advantage. If you compare the diesel F350 with the F450 the F350 has only a small advantage (especially when the F350 is ordered with the wide track option).
As to why the F350 and F450 have the same payload rating, you would have to ask Ford that though most of the speculation I have seen revolves around federal rules applying to vehicles over 14.000 GVWR.