I wager what they actually want to do is treat those without an account the same as those who are not logged in, but realize they can't get away with that quite yet. (That is to say, I suspect that they would like to turn away all users without accounts.)
I'd assume (and I have no insider knowledge) that it's because Google assumes if you have a Google account that you probably want to be logged into it when using Google products. Otherwise it would suck if you tried to do something that requires an account and were told "log in" and then had to refresh the page, potentially losing state. The up-front login avoids some usability issues there. (Of course there are many ways to approach this, this is merely my hypothesis.)