The "14-days abroad" limitation for free users might be the issue. Perhaps people were just creating 2nd and 3rd and 4th etc accounts.
Aside from this, there are still valid technical reasons to abandon local login. Just because you have a capcha, forgotpw, etc system written doesn't mean there aren't benefits from getting rid of it - including the portion of the support staff responsible for "I can't log in" problems. And if they used multiple OpenID providers? Expect to hire more.
I'm not saying that this is the reason Spotify is doing what they did - I don't work for them. But it's certainly plausible that going FB-only is being done for technical reasons, and I expect they have a pretty good idea of what they're gaining and losing by making this decision.