Why should Barack Obama, or any other politician, have to DENOUNCE or REJECT any of their supporters?

It's so that people realize that the politician doesn't support or endorse the person supporting them. Obama did very well denouncing and rejecting Louis Farrakhan, and Hillary sould do the same for Coulter. Most people know Obama and Clinton both are not anti-semitic, but some people just want reurance. I would've still voted for Obama either way. I know he doesn't support Farrakhan, he doen't have to reject and denounce him to tell me that. It doesn't bother me what people wanted him to do so though.