Francis Bellamy was a socialist nutjob obsessed with preventing another attempt at secession: one nation, indivisible...It's like he's rubbing the South's face in it. Is the "indivisibility" of our "one" nation anywhere in the Constitution? Are any states currently trying to split? It's as relevant as a pledge that insists on our autonomy from Britain.
I think the difference is that we are pledging our allegiance to our country that believes in liberty and justice for all...
... versus in North Korea, Cuba, Iran, China and others where you are pledging to the person whose picture is posted anywhere you might look throughout the country.