For me, it's because their cult puts its confidence that salvation is inextricably tied to being a devotee and obedient to a group of men, and conformity to changing rules. Not just to their acceptance to men or a religious group, but stating that such obedience is required by God for salvation, notwithstanding that their requirements and stated understandings are relatively fluid and sometimes difficult to keep up with, as well.
Christians are supposed to follow and put their confidence in Jesus, not just pay lip service to following Jesus while jumping to do the bidding of men who require things of them not even suggested by Jesus.
They are not alone in denominational salvation teachings, but they are not even subtle- and many churches are promoting Samaritan type work while JWs mock it, mock mercy and mock charity in general. How that is Christian is beyond me.