You can login to Spotify via Facebook as you can see in the web version: https://accounts.spotify.com/en/login/ For doing that in a app they require the SDK, which then does all the spying for Facebook.

Why is the Facebook integration not lazy-loaded only for users who log in through an FB account?

Because FB doesn't support that. Either you build it in and it runs some code when your app starts or you can't use it.

I would vote for "not use it" but others want Facebook integration supported and happily share information on all their users to Facebook ...

P.S. I believe part of this is also Apple, who don't like runtime loading of code, as that makes their verification harder, but I'm no iOS dev or user.

You can share what you're listening to on Facebook

"Lewisj489 is listening to Band on Spotify"