The definitive answer (with copy of Amazon email) is below. I'm an international user who used a valid US address but change country code to access a couple of titles only available outside the US. I think the frequent changes raised a flag at Amazon and the email is the result.
Hello from Amazon.
We are writing because the home country registered to your Kindle account may not match your country of residence. Due to publishing rights, the home country registered to your account must match your country of residence.
To continue purchasing titles available for the United States, please send a copy of your valid government-issued identity card, passport, or a utility bill received within the previous 90 days to our secure fax line:
Within the US: 206-266-1838
Outside the US: (00)1-206-266-1838
For your security, you may obscure the passport, ID, or account number. However, we ask that you do not obscure any portion of your name, your address, or any expiration or statement dates.
Your fax will be converted to a secure electronic image that will never be printed. Once we have completed the verification process, we will delete the image.