Good question - I have no idea. But if I google 'gas-filled tube ethernet', I get a number of hits like http://en.tdk.eu/blob/174150/download/5/smd-surge-arresters-... and http://www.first-electronic.com/uploadfile/2010989552637708.... which suggest that these products are sold for networking purposes to prevent surges. Since this is in a military context, I would hazard a guess that this may be some sort of standard hardening requirement to try to protect the datacenter against lightning strikes, EMPs (such as from nuclear strikes), and general accidents. This may sound paranoid, but then again, so do Faraday cages, and it is the military - it's their job.

Other responder might be more right, but I was thinking about how hard it would be to discreetly tap into a ethernet cable when I have to break glass.

You can pressurize the gas and put a barometer in the tube in order to detect any breach of the tube. This was used during the cold war and the NSA tapped the communications cable anyway by filling the space around the tube with gas as well. Source: James Bamford's books on the NSA.