Sharpton has a long history of controversy, starting with Tawana Brawley case, which is largely accepted to be a hoax (here and here). Sharpton was accused of slander in the aftermath of the case and was successfully sued, but Sharpton refused to pay up. Sharpton's portion was eventually paid by others. Sharpton has also been accused of some pretty shady business and political practices as well.
Maybe it's just me, but I think that the President of the United States should be held to a pretty high standard in who he speaks with and what groups he speaks to. With some liberals trying to play the association game with politicians like Governor Scott Walker and with people like the Koch brothers, I hope they hold the politicians of their party to the same standard.
Additional commentary: Jonathan Turley's legal blog.