I am using a custom ROM, so things may be different... that said...
The button for emergency call only appears if you have some type of 'security' on your lock screen; PIN, password, or visual. I wouldn't really call the 'slide to unlock' a 'lock screen' per se, so much as a 'preventing butt dialing screen.' Why would such a screen need an emergency button? If you can't swipe your finger across, you probably aren't going to be able to dial 911. I would think the whole point is so that if the phone's owner is incapacitated, someone else can use the phone to make an emergency call without the PIN/password.
Now, if you're not getting the Emergency Call button and using a PIN/Password... then something is wrong.