I think this comment has the right idea [0]. Amazon has two problems with seeds:
1. people doing what appears to be a weird scam that involves seeds being mailed across boarders
2. seed sellers on amazon have been flagrantly violate import controls for years.
This new policy seems to be addressing #2, which is important to Amazon now more than ever because of the increased scrutiny caused by #1 making international news.
I've personally bought seeds from Amazon which were declared as "bookmarks" to sneak past US customs.