So are you going to go after the makers and vendors of World War II newsreels that do precisely that? >:*3
Saying "One nation...indivisible" works as a self-contained phrase much better than "One nation, under God, indivisible" does. The latter requires a continuation for the prosody to work [unless one were to add a word to make it "One nation, under God and indivisible" which would be just plain ugly.
You can't leave out what isn't there. 'Under God' was not part of the pledge in WWII, although that straight arm salute which is so distasteful now was part of taking the pledge.
Funny you should mention that. A few months ago there was an outcry on FR about an old Looney Tunes short wherein Porky Pig recites the pre-1954 pledge. Even though he was old enough to know better, he swore that the short had been reedited to remove the words "under God". Others insisted the same thing.