the displays do not operate standalone but bolted into the cockpit instrument panels and are not affected in the same way.
This is true. But the certification process has to account for an aging fleet. Where the grounding path between cockpit panels and the airframe may be degraded by dirt, oxidation and repeated removal/reassembly for maintenance. So the system design gives no credit to the shielding characteristics of cockpit structures. The alternative would be a repair and reassembly procedure that would include cleaning and treatment of each panel and LRU to ensure proper RF shielding. Plus a procedure to check this following any maintenance. So the DUs have to operate in 'free air' even though the structure does provide some level of shielding since it isn't an easily controllable factor.
