Is it just me or is the law unclear on this issue? The statement below suggests the CBP may ask to see a permission note, but under the link you provided they say it is not required in the US?? Which is it ??
The law is that permission note is not required but if they suspect anything they have the right to ask for it. Hence, there are stories about people never having issues at the border and stories of people who had troubles. That is exactly why I always recommend that my clients carry such a letter with them. However, it is not required by law in the US and Russia.