Okay well in the black community crimes are seen as a way to gain financially. This is due to the socioeconomic environment in which a significant portion are raised and the cycle of poverty in the black community. So for instance a black kid with no job and no options may chose to rob a bank or a girl may chose to prostitute herself to make a living. Stereotypical of blacks, crimes arent derived from much other than gain. A black guy will shoot you but if he does its because he stands to gain some advantage from it. Listen to hiphop. All that talk about snatching your chain and what not. The DC snipers however were committing crimes with no apparent gain. No financial benefit to be had. This made the crimes seem completely nonsensical in that regard. That being so it is thought that whites tend to be more psychologically unstable than blacks. Leading them to be serial killers, child molester, and due to the social stigma in the black community even homosexuality is seen as a disease of the mind. So the DC snipers seemed to us(blacks) to be white because of the pattern of behavior we expect from whites seems to coincide with random killing without rhyme or reason.

TL DR - I thought white people be crazy. Killin random people aint makin you no paper. Why kill people for no gain? Only a white guy would randomly snipe people for no reason.