My '14 was the exact same way.
The Dearborn plant assembled the trailer hitch so high up that it was actually contacting and rubbing on the truck bed and causing the rust.
I suspect that there are many Raptors that have this contact when there should be a small gap.
Do-It-Yourself fix:
Loosen the 6 trailer hitch bolts to drop it down (the bumper will also drop as it is connected to the trailer hitch).
Don't worry it will only drop about 1/2".
Then clean up all of the rust. Buy some white krylon or similar and paint it.
After it has dried, lift the trailer hitch up on one side (gap should be approx 1/8" from bed) and snug up one bolt.
Go to the other side of the Hitch and lift it up so the gap is approx 1/8" away from the truck bed and snug up one bolt.
Get out from under the truck and stand back and check to see if the bumper is level.
Adjust as needed to bring the bumper level and tighten all six bolts down to 129 lb-ft.
NOTE: When you bring the bumper to level and tighten everthing up, do not expect the gaps to be equal on the left and right hand side.
Some of these trailer hitches have been welded together on a Monday or a Friday (If you know what I mean).
You may have a gap of 5/16" on one side and 1/8" on the other side. But your bumper will be level.
My trailer hitch was welded together so bad that I am now in contact with a Ford customer care representative.
My trailer hitch needs to be replaced with one that was welded correctly.
OR.........
Drive down to your local dealership and show them what the Ford Dearborn factory had done and that you would like to have it fixed.
I encourage folks to let Ford know of this problem.
With my bumper level this is the gap on the left and right side on my Raptor now.
These are the trailer hitch fasteners. 3 on the left and 3 on the right.