You might think that a threat against the actual sitting President might be more newsworthy than a threat against a presumptive presidential nominee, but not according to the headline or the article.
If this was done purely because Obama is black, then the article is racist and surely the post-racial Obama would denounce the implication... Of course, then he would change his mind. Next he would say that this is not the Associated Press he knew, and finally, he would throw the AP organization under the bus.