They arent. But like any liberal astroturf, Whites run the show and use brown people as cannon fodder.
The face of BLM is a white guy. there are many examples.
True that they are not limited to whites, but that movement does seem to be largely made up of upper middle to upper class white people educated beyond their ability and with too much time on their hands. Homosexuality is generally VERY much frowned upon among Blacks.