They need to figure out something for shared Prime members. They let families share Prime across multiple accounts, but then restrict stuff like this just to the main Prime account. It is absurd to think a household is going to pay for multiple Prime memberships.
They don't appear to be trying too hard to block people from doing additional trials. I just signed up for another month-long trial using a different email address and a credit card that has an updated expiration.
My aunt interviewed for an executive position there in 96, 97, and we all told her not to pursue it because they were headquartered in Minnesota (which sounded like the middle of nowhere at the time) and we figured they wouldn't last. She ended up doing just fine and is very successful in another industry, but ... still. I wonder if the opportunity would have been better there, at the ground level of Amazon.