Their danger is beyond refusing to serve in the military or to vote--if it were just that, I would see no problem. However, they initiate death threats against anyone that does not join, quoting the Bible as the prime authority (itself a problem, since the Bible itself lies). Worse, they take it out of context (initiating fraud), in order to support their beliefs that anyone not joining will die at Armageddon (a death threat). It is this death threat that gets many people interested, along with the fraud of a paradise earth.
Then, once you are in, they coerce you into staying under threat of breaking up families. If you refuse to go out all the time, you get hounded to death (and often, they will insinuate that you are not going to survive, a death threat, unless you max out your field circus. They usurp all your time, preventing you from working to the benefit of self or society. And there are so many good careers that they forbid members to pursue. All of which leads to poverty (especially when they have those sell-out drives, as they are currently doing), which further drains the economy in the state. So far, that is enough to get the religion banned.
Worse, they misrepresent themselves in court to get themselves legalized. They claim that children have normal careers (the truth is, their career is to pio-sneer or Beth Hell). They claim that they put no pressure on people to join, and that they join under free will (like a death threat amounts to free will) and stay under free will (a death threat plus disrupting families, and the endless hounding). These lies that are used to make themselves look good to the public, while they are a group that initiates death threats against members and the public, and defrauds people into joining and doing ever more.
All of which is worse of a threat against a free society than not taking blood and banning voting and military service.