Since I'm the Director of a 9-1-1 center, I know a little about this

...if they don't call back within five minutes, there probably isn't a need. The idea behind it is most cellular calls (when the emergency call back mode was put on the BB) was used in vehicles.
Now, a large percentage of cellular users use their cell phones as their home phones so it seems like this feature could be removed but it's there when needed and you are able to manually override it if you need to make a non-emergency call or call your voicemail to deactive the emergency call back mode and start receiving messages again or access the internet.