I don't view the choice the parents have made as bullying, coercion or blackmail. They have made a decision to disassociate from their adult children based on their convictions. Everyone, in a free society, has the right to associate or not associate with the adults of their choice. Moreover their tactics do work to the ends of their cult whether people protest or not. Shouting, "That's not nice!" is not going to change anything.
Best let them isolate themselves.