The suspension may be the same in back, but it is certainly different IN FRONT.
There's 200-300lbs of 4x4 "stuff" up there that's almost entirely carried by the front axle, which is designed to carry the extra 200-300lbs.
As others have noted, the rear end weight is virtually unchanged, and cargo is carried almost entirely by the rear axle. Thus it is perfectly valid to raise the GVWR by 200lbs on a 4x4.
That said, people looking at 1/2 ton pickups rarely care about cargo carrying capacity and don't even understand what the GVWR is. It's a truck, it will haul anything.