I'm surprised that Facebook supports them(and yes, they really want Facebook password)As of their policies, it should be disallowed to request password from user'sYou must not include functionality that proxies, requests or collects Facebook usernames or passwords.So what is going on ? Is that just some exception? 
Does Spotify REALLY want my Facebook password? Isn't it against Facebook's privacy policy/TOS?
spotify
This would normally be cause for alarm and against Facebook's TOS, but in this case Spotify and Facebook have entered into a direct agreement.See: TheRegister's acticle and one at Forbes for further detail.