He wasn't speaking "to ACORN". The Heartland Democratic Forum was a meeting of a variety of non-profits and community activists which all of the major democratic candidates spoke to. He told them that they will have a voice. Why shouldn't they? What, only rich white people should be heard in the White House? That is the implication here, is it not? This was not a forum of evil people. These are grassroots non-profits trying to help people.
I know it's scary because there were a lot of people of color in the audience. Get over it.