Often times it's about the nazi (or more correctly national socialist) aspect of JoS, which people in general have difficulties with, and even more so than Satanism per se.
When I was totally new to JoS and before I became more hooked on it, I had some doubts about JoS becuase of this particular (nazi/antisemitic) aspect. People on some other 'Satanic' forum wrote that JoS was "pseudo Satanism" with the only goal of brainwashing young people to become nazi supporters, which is not an unusual claim out there among people who don't know any better, or who don't have the intelligence to understand JoS and how nazism fits together with paganism.
Speaking of brainwashing; the enemy is the one trying to brainwash us all into thinking both nazism and Satanism (or paganism) is evil. As HPS Maxine has stated though; it's a sad thing that one has to do a ton of research to get to the truth.
Most people are too brainwashed and lazy to really do so, an thus you see them slander JoS.
I later on read more about national socialism and even joined The Nordic Resistance Movement for a while as a supportive member, went on their lectures and demonstrations etc.
But I've always been more of a JoS member than NRM member (which have some members being catholic etc.), even though I support the idea of race separation and national socialism (which also translates to people's community) and understand how it's connected to Spiritual Satanism (the third reich being anti-christian and very pagan with all the ancient symbology, ceremonies etc.).