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.)
With some insider knowledge: Because this document is commentable for (some) Googlers, though you don't see the comments on the /pub version that is accessible externally. Docs would rather force you to verify your state (Logged in or anonymous) than having some things 'not work' because you're expecting permissions from login cookies you don't have.
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.)