The government is not asking you if you are a member of a religious community that has not choosen to use that service.

I trust my government with my data (well, mostly) because if it abuses that trust, I know that I and my fellow citizens can vote it out of power. We have no such control over FB.

Afaik you can enter whatever you want on the tax form, it is just used to collect membership fees/church tax on behalf of a church. Mostly part of an old deal between the state and the catholic church.

Scientology on the other hand counts more or less as an enemy of the state, stability and not something you let near children. So you need to fill out a corresponding form for specific jobs. If even half the stories of what they get up to are true that is a good thing.