Google Drive saves your login credentials, or at least some sort of authentication token, otherwise you'd have to enter them every time at launch.

Naturally, anyone with access to your computer could use those to access your Google Account. Google just made it more convenient for you, the authorized user, to do so, by adding this feature. It does not reduce security in any way, as even without the option to log in to your account on the web, the authentication information for Google Drive will still be on your machine unless you want to log in every time.

Of course, they could use a special authorization token _just_ for Google Drive, but that's not how Google's services have ever worked.