You said it yourself. They already have this information.
You are absolutely right in that there is no good reason for the government to take an interest in you. However, the reality is that the government is full of people who hate people like you.
Do you think morality is a higher authority than the law?
Do you think it's your choice what you think?
Do you think it's okay to carry a loaded pistol around?
If you answered yes to any of these questions you are a threat to the new government. The agenda here is state dependence. This is, believe it or not, much bigger than guns and that's pretty big.
They already keep track of how many guns you own. Why on earth would anyone honestly need to know that?
You are projecting your logical thinking onto the non logical mind of your enemy. These people want to decide what kind of health care you receive, what you think, how many kids you can have, what you believe, and when you're no longer of use to the state.