There was a thread a while ago with a google engineer taking bug reports on this. At the time both iOS and Android browsers were redirecting to a login page (on Android you could request the desktop version of the page and it wouldn't prompt you to log in). Since then you no longer get redirected on Android, but maybe it hasn't been fixed in mobile safari?
Facebook, on the other hand, tells me that I can't continue if I don't log in (although I can just cancel the message, and continue reading).