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.
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.
