I imagine Mozilla's thinking there was that they should position the feature so that people who already use Pocket will realize that Firefox isn't adding a separate reading list (like the Safari one nobody uses), but rather that you can just sign into your existing Pocket account in Firefox, and see your existing Pocket reading-list.
But yeah, in the end that probably wasn't nearly as important as getting people who didn't already use Pocket to see the reading list as "Firefox's reading list" rather than some channel-partner bloatware encroachment.
A happy medium would probably have been if the Firefox reading list was its own skin of Pocket, and synced using your Firefox Sync account, without needing to create a separate Pocket account; but when you first went to use it, it would ask if you want to sign into your Pocket account; and if you do, then your Pocket account would be merged with your Firefox Sync account, because "Pocket Sync is now part of Firefox Sync."