I know, and that's the first thing I thought of too when I found out that they are structural. Apparently, to keep the body properly stiff with modern thin sheet metal and all the openings, additional superstructure is required.
The Convert-a-Cab system meets that market demand, but it also creates an engineering problem: how to restore the body's structural integrity lost when the SUT's rear wall became a door. The answer is the addition of boxed steel reinforcements throughout the rear section of the vehicle tied to the solid member of the C-pillar structure by the sail panel. Gjestvang says that the engineers came up with the sail panel's 45[degrees] angle to "tie everything together as a structural system." From there they worked with the designers to achieve a part that was both structurally sound and aesthetically pleasing, and ended up creating the signature design cue for the vehicle.