This is a weird move. I think if I didn't already have prime, I'd just go somewhere else rather than be motivated to subscribe. Hopefully this is just an experiment—I suspect this is ultimately going to be self defeating.
By forcing people to consider alternatives, it is making people experience that initial friction elsewhere, but once they are setup there the friction is less. It won't take very long to discover other retailers have better prices, better shipping and less quirky sites.
Essentially this is nice gift to Amazon's competitors. Amazon will find out just how "sticky" those competitors are, but this stills seems silly.
https://www.amazon.com/forum/amazon/ref=cm_cd_notf_thread?_e...
I'm not saying this is their reasoning, I'm just saying it could make sense.