That's not because of who he is. It's because of the city which he is "reppin"
For those who aren't as baseball savy as myself (GO SAWX!), Chicago and St. Louis are big rivals. Now I know it's Cubs vs. Cardinals, and not White Sox (Obama's team) vs. Cardinals but the point is that the two cities are rivals.
Also consider this: the all-star game is not the average ball game. The tickets are VERY expensive, and that means that you have a lot of rich white folks who do not very much enjoy President Obama's economic policies. Also, the crowd was probably 60-70% cheering, only that those who were booing likely were a lot louder. Also it's in Missouri, so there's that.