Why do you want me to sign in with Google?
There's no free lunch. The OAuth providers are, on balance, pretty good at what they do and a better choice for account management at small sites. This one happened to pick Google.
Yes, either this or openid. I don't have an account at google. I don't want an account at google. While it's a lower barrier than a facebook login, it's still enough that I'll bounce off the site when I see it.
Of course there is. No sign-in at all.
My question should have been: Why do I have to sign in?
Presumably they are storing that information in their back-end somewhere.
I choose Google because I didn't want to worry about email verification, salting, key derivation functions, etc.