And yet that in no way gives them the right to use that to log into your facebook (rather than using the API to authenticate) and install the spotify facebook app (giving themselves whatever permissions they like without asking the user).
Of course, the lesson here (besides that spotify cannot be trusted) is that you should never use a password for more than one account. Sure makes me glad I switched to using a password manager that uses randomly generated different passwords for each service I sign up to.
Facebook and Spotify are tightly partnered together - at one point they actually REQUIRED the use of Facebook to log in. The option to register without Facebook was only reintroduced recently.
For Facebook they do. They have a tight partnership. If Spotify did something wrong with their Facebook app, Facebook would have removed their app a long time ago.
In the US, when spotify first opened up to Americans you could sign up without facebook credentials. About a month later, they decided to go facebook only for new setups.
Spotify started requiring a Facebook login from all new users on 22nd September 2011. On 6th December 2012 they put the non-FB signup option back. You signed up before they required FB so you never saw the requirement.
If you read the reply again, they allowed you to use a username, then required facebook, then reintroduced the ability to user a username. You probably were using a username, before they started requiring facebook.
As far as I know, spotify uses username for login, and not email. By using your email, it's assuming you want to use facebook.