Comment 36 for bug 878836

When you're at the lock screen, sound keeps playing as it should. And the microphone and camera keep working as they should. It would be a pain if a Skype call, for example, was effectively cut off just because the screen saver momentarily came on. Any accessibility options you were using during the session should also be in effect at the lock screen, because it's your session that you're unlocking, and ignoring them would be gratuitously annoying. And on Touch, apps like the camera may be available in a limited way at the lock screen, and when they are, they should store data for you and not for any other user.

When you're at the login screen, on the other hand, sound output is cut off as it should be. The microphone and camera should also no longer receive input. Any accessibility configuration should be system-wide, because it's not necessarily you who is logging in. And on Touch, apps that store data can be available only if there is one user account. (In theory you could select a user and then launch an app before logging in, but selecting a user already prompts you to log in and may even log you in automatically, so there's barely any point.)

This makes the greeter fundamentally different from the lock screen in many non-visual ways. So the answer to your question, Bence, is that this feature was not completely designed. For a long time it was treated as merely a visual design problem, without these other issues being tackled.