Orbital transfer vehicle design question

[Zatnikitelman]

When looking at long-range transfer vehicles, like could be used for heavy transfers between LEO and the Moon or LEO and Mars, I've noticed one peculiarity in the structural design of such modules. Most follow a design of engine > tanks > open bay > crew module. The peculiarity is that in virtually every design, such as this one: https://upload.wikimedia.org/wikipedia/commons/4/4c/Orion_docked_to_Mars_Transfer_Vehicle.jpg there is some kind of open-lattice structure on "top" of the bay, meaning it truly functions like the shuttle's payload bay and is only accessible from one side. Why? I don't see why the two side rails couldn't be designed to be strong enough to absorb whatever loads that top structure is designed to take.

 

[Pipcard]

For that Mars Transfer Vehicle design, the fuel tank was designed to be jettisoned from that one side after performing the Earth Departure Burn. The lattice structure is for additional reinforcement during launch.