There might be some more detail or clarification needed in this question.  I think that Douglass gives the speech about how the concept of "Independence Day" does not apply to African- Americans to highlight the fundamental difference between America's promises and ideals and its practical reality.  Douglass was precise in pointing out how the institution of slavery's existence undermines each of America's promises and possibilities.  The notion of Independence Day, a moment to commemorate the freedom of America and the birth of a nation conceived in liberty, cannot apply to Africans and African- Americans who are enslaved.  Douglass' harsh rhetoric and tone is meant to provoke the reader into action, to be able to fully comprehend how there is a difference between theory and reality of American government with the presence of slavery.  Douglass' speech concludes that until slavery is abolished and completely removed from the American lexicon, moments like the Fourth of July, meant to galvanize Americans into a collective and shared consciousness of glory and pride, will be sources of shame and repugnance for those who suffer under the heel of the very same people who extol the nation's independence.