Census participation is not voluntary. Failure to provide complete or accurate data is, in theory, punishable by a fine. Last census, I intentionally provided incomplete data on the web form, which resulted in a person with a clipboard and some stern questions showing up at my door.
To counter this, I had a census taker show up at my door and ask me if they knew how many people lived in the neighboring apartment. Where was the consequence there?
I don't see what that has to do with whether census participation is voluntary or mandatory.