On the surface of things, America is a country without classes. According to the story of the American dream, people can become just as wealthy and prosperous as they want to be, provided that they work hard enough. Think about that idea for a minute, though. Is it really true? Unlike the country America, the poem "America" is all about raising class-consciousness and championing social underdogs. The poem's speaker wants to help those who might get stepped on, or over, by the more successful members of society. He's like a hall monitor, making sure no bullying gets done on his watch. In this case, though, he's got his eye on the whole country, not just one school.
Since he himself occupies the lowest social class, the speaker's complaints about America seem more selfish than selfless.
The speaker's protective attitude toward the underprivileged class stems from the fact that he believes it's the only way to ensure justice and equality in a capitalist society.