I am reading Churchhill's account of WWII, and was impressed by an episode that he relates wherein he was driving with FDR and co. to the cabin that would become Camp David (then called Shangri-La by FDR). Harry Hopkins dropped the first two lines of the poem, then Winston picked it up the ball ran it in, and spiked it.

But in his account, he then he goes on to heap praise on Stonewall Jackson and Robert E. Lee as "two of the noblest men ever born on the American continent".

Barbara Frietchie was a Unionist, and the poem is about her waving the Union flag to taunt Jackson's rebel troops as they marched by. Whittier was an abolitionist.

I guess Churchill didn't want to mislead the reader.