They are racist. Therefore, they are right wing. That's how the liberal mind works. No need for anything remotely resembling proof.
Even calling a white prison gang a white supremacist group is a stretch, much less suggesting that they are "right wing extremists".
White guys in prison join a white gang as a self-defense mechanism, to protect themselves from the Hispanic prison gangs and the black prison gangs. It has little to do with "white supremacy" per se, though lip service might be paid to the concept.
But, you're right, the "racism" angle automatically makes these groups "right wing" to the liberal media -- even though they're nothing of the sort.
Perhaps we should start referring to black prison gangs as "left wing extremists". Or "ultra-liberals"...