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.
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.