Originally Posted by
N674UW
Not entirely correct I'm afraid, military airfields in the US often times start with N(but not always!), and W seems to be somewhat common as well:
A couple of N's:
NZY - NAS North Island in California
NTU - NAS Oceana in Virginia
NXX - NAS/JRB Willow Grove in Pennsylvania
(there are many more)
Some W's:
WRI - McGwire AFB in New Jersey
WDG - Enid Regional Airport in Oklahoma
WYS - Yellowstone Airport in Montana
Ns are not prohibited, they are just reserved for use by the Navy. Hence all the Naval Air Stations in your list.
I know of multiple W airports as well, so I can't vouch for the validity of that rule. It could be something they used to block, but now allow. There are far fewer Ks than Ws, and those would have been blocked for the same reason (FCC reserved for radio stations).