Though we may see these African Americans as "men" today, pro-slavery Americans did not see them as that then. They were beasts, inhumane creatures. They were stolen, sold, and forced to work for little or no compensation. .
"Behold the practical operation of this internal slave-trade, the American slave-trade, sustained by American politics and American religion. Here you will see men and women reared like swine for the market. You know what is a swine-drover? I will show you a man-drover. They inhabit all our Southern States. They perambulate the country, and crowd the highways of the nation, with droves of human stock .These wretched people are to be sold singly, or in lots, to suit purchasers. They are food for the cotton-field, and the deadly sugar-mill .Attend the auction; see men examined like horses." (1890).
This is how slaves were treated, because they were not seen as men, and they had no rights. So were they Americans? They lived here, worked here, slept here, died here. But were they here as Americans, or just as cattle, swine, or dogs? According to Douglass, they weren't Americans. He was not an American, though he believed he should be. "What have I, or those I represent, to do with your national independence? Are the great principles of political freedom and of natural justice, embodied in that Declaration of Independence, extended to us? and am I, therefore, called upon to bring our humble offering to the national altar, and to confess the benefits and express devout gratitude for the blessings resulting from you independence to us? I am not included within the pale of this glorious anniversary! Your high independence only reveals the immeasurable distance between us. This Fourth [of] July is yours, not mine." (1886-1887).
To Douglass, American identity did not include slaves or even freed blacks. You had to be white to be American. He was not white, therefore he did not see himself as an American, and was offended to be asked to help celebrate the independence of America.