Some possibilities:
- Doctrinal: they claim to be Christian, but they deny that Jesus is God and the Holy Trinity.
- They have a reputation for wanting to share their religion, and we really don't want to talk religion, particularly when we might disagree.
- They are unpatriotic. Besides refusing the more symbolic stuff like flags and anthems, they won't participate in democracy or in the military.
- They refuse blood transfusions. They are dangerously anti-science.
