I don't know why you're confused. Some games do not save your account information at all server-side, despite the fact that you can (and usually do) purchase items through microtransactions. This means that if you ever delete the game from your device, all progress and purchases you made are gone forever.
So imagine you bought a game on your Xbox and the DLC for it, but if you deleted it you lost that DLC forever. But it's actually worse than that when it comes to loot boxes, because
1) there's no limit to how much you can purchase, making the need to keep it on your device more insidious, and
2) you can't even purchase your items back 1-to-1, so would need to buy a ton just to (maybe) get back what you had before.
To be clear, I'm fine with games that are single player and don't have MTs not having cloud/server-side storage. But when you start charging for things in-game that are lost when you delete the app (or your device is broken), that's when I start to have an issue.
Hopefully this clears things up.
Either way, not sure this is the right thread for it as I haven't played FFRK and don't know if it has this specific issue (although, like you said and I acknowledged, it does have a FB option.) I meant this more as a quick aside, as it reminded me of issues in similar mobile titles.
You're conflating something you don't agree with onto something else you don't agree with just to beat a strawman over the head with terms like 'insidious'. PAD has had a similar account migration option since 2014.
Plenty of non-gaming applications allow you to log in via facebook as a convenient alternative.