I don't think it's fair to blame the user for that. This is a standard-looking login form that users will have seen hundreds or thousands of times before. You don't reinterpret the words on a login form every time you see a new one; you type in the stuff to log you in without really thinking about it.

Regardless of Spotify's intentions here, they're benefitting from users' trust in normal login processes to get Facebook account access. Lots of designs exploit users' automatic behaviors like that; see Dark Patterns [1].

[1]: http://darkpatterns.org/