Celebrities have an outlet, for better or for worse, to publicly address millions of households. Normally this information is entertaining or irrelevant to most people's lives but occasionally, depending on the prevailing discourse, certain topics can be taken to extremes and cause harm on a wide scale.
In the last several years diseases like whooping cough, measles, and other have returned due to the "Oprah-effect" of giving bad science a public forum. While not illegal, it's highly irresponsible and should be addressed by the highest office to lend the maximum amount of credence.
Please, Mr. Obama, publicly denounce the anti-vaccination theorists and the bad science they spread.