Seriously though, I don't quite understand why anyone would limit the cargo carrying capability of an open truck bed. Sure, it'll provide protection from the rain, but they can also get in the way and they're not that easily removable....
To keep your friend from making u help move em again.
To keep mattress clean for friendly natives.
To keep your beer out of the sun.
So people understand why your driving 40 in the fast lane.
That subconscious desire to be a turtle.