Comment 3 for bug 446218

Because it's a non-issue. The screensaver is responsible for locking the screen, and you can kill it just like any other process. However, just like any other process, it can't be killed by any other unprivileged user other than yourself. If the screensaver wasn't responsible for locking the screen, then some other process would be instead.

If you leave yourself logged in to a TTY as well as a graphical X session, then it's just the same as leaving the screen unlocked (whether an opportunist kills the screensaver or not, they can still access all of your files and personal information, and start a graphical session on your behalf)