I share your curiousity! When I purchased my Alu Cab Shadow, I didn't look real close at the back or vehicle side of the wall kit and just assumed there was a back wall. Why else would there be screens in the windows??!!
I asked Rin at OK4WD why there were screens and he checked with Alu Cab. Their lame answer after several months waiting was, it helps keep the structure of the walls intact. ? Also, their reason for no back wall was that there are too many variables of rig doors & tailgates to provide generic access openings in the back wall.
Anyway, needless to say, there is very little pest protection provided by my Alu Cab wall kit with no back wall and it would be the same with any brand that doesn't provide a back wall, as many potential customers have noticed. The only 270* awning wall kit that I've seen with a back wall is the Foxwing. But it just doesn't compare for strength in the wind.