Most secured cards have no rewards and many have a yearly fee. If you have prime anyways it's an amazing deal.

Sure, if you weren't otherwise gonna get Prime, you'd have to spend $2400/year to break even. If you're one of Amazon's millions of Prime subscribers, it's a no-brainer.

The target demographic might overlap with the set of people who are candidates for discounted prime membership.

Plus 28% APR, though I can't imagine holding a balance on a secured card.

Secured cards are held by irresponsible people who carry balances. Otherwise they would get unsecured cards.

Holding a balance does not mean you're irresponsible, it just means you will be paying interest.