It probably wasn't, but we needed some way to save high scores, and "outsourcing" auth to FB was the simplest way. I've done other projects where that's optional, and that'll probably be the end goal for this as well.
If it's any consolation, I don't like sites that only use FB auth either... this is just a product of the time-constraint.
I agree that they probably didn't need Facebook, but a generation ago, it would have been an email address that they required, and I would have just used a "junk" account. I'm honestly wondering, for people who don't like Facebook, what is the problem with just having a "junk" Facebook account?
you can still have junk facebook accounts, the sign up process is slightly longer than hotmail's, but not by much. I still have around 15 junk facebook accounts or so... I only remember the passwords for about 3 of them.
If it's any consolation, I don't like sites that only use FB auth either... this is just a product of the time-constraint.