In the terms, there are 2 separate credit-builder cards. Only one requires Prime. The one that does not require Prime is not eligible for 5% back UNLESS the cardholder also has Prime.

> 4. The Amazon.com Store Card Credit Builder. The Amazon.com Store Card Credit Builder is available to customers with an Amazon.com account, subject to credit approval.

[...]

> 5. The Amazon Prime Store Card Credit Builder. The Amazon Prime Store Card Credit Builder, an upgrade from the Amazon.com Store Card Credit Builder, is available to customers with an Eligible Amazon Prime Membership only, subject to credit approval.

I didn't dig into the FAQ or fine print, but I wonder what happens if you get the card, and then your Prime membership runs out?

EDIT:

> The 5% back benefit may apply to purchases (less returns and other credits) made using the Amazon.com Store Card Credit Builder when signed into an Amazon.com account with an Eligible Amazon Prime Membership, including 1-Click orders and purchases made at physical Amazon locations (in each case, where 5% back is selected as the default option or where Promotional Financing is selected as the default option but is not available for the particular purchase).

It seems like the 5% back is only for Prime membership folks. I think anyone can get the card, with or without Prime, though. So then it doesn't have a yearly fee, technically.

There is no fee. Having prime gives you the perk of 5% cash back but you don’t need it to get a card.