I tend to agree it doesn't make a lot of sense, but it's not much fun to get locked out of playing one side or the other.
I like the letter of marque analogy as a rationalization; you should be able to hold ranks with both as long as you can avoid ing them both off. Relevant quote from wikipedia article on the subject:
A letter of marque and reprisal in effect converted a private merchant vessel into a naval auxiliary. A commissioned privateer enjoyed the protection of the laws of war. If captured, the crew was entitled to honorable treatment as prisoners of war, while without the licence they were deemed mere pirates "at war with all the world," criminals who were properly hanged.
[18]
For this reason, enterprising maritime raiders commonly took advantage of "
flag of convenience" letters of marque, shopping for cooperative governments to license and legitimize their depredations. French/Irishman
Luke Ryan and his lieutenants in just over two years commanded six vessels under the flags of three different nations and on opposite sides in the same war.
[19]
http://en.wikipedia.org/wiki/Letter_of_marque