Why do I have to declare whether I am a US citizen?

Tax residency in Norway and most other countries is determined under rules that have criteria associated with domicile and/or place of residence.

In the USA, the tax system is based on both tax residence and citizenship. US citizens are therefore liable to pay tax in the USA on the basis of their citizenship, and Norwegian financial institutions must therefore record information regarding whether customers are US citizens.

If a financial institution you are a customer of has information to suggest that you may be a US citizen, you may be asked to confirm or deny this.