I imagine this has to be a loss leader/marketing feature, where the hope is very few will use it but those who do will result in "iPhone saves 12 year old Boy Scout lost in the woods!"
Loss leader isn't the proper term in this case. Think of this akin to the health-scare clips they've been showing in the past few years when talking about the Apple Watch (like today).
The number of Afib/high heart rate detections are miniscule, but serves as great marketing. Good enough for me to order one today.
The Apple Watch doesn't currently check for heart attacks, though, and makes a point of telling you that every time you use any of the heart-related functions.
It can detect atrial fibrillation, but that's a different thing from a regular heart attack.
1. This feature is relatively expensive for Apple. Satellite communication isn't free, and Apple don't have their own infrastructure for this.
2. They've signed a 2 year contract with whichever provider they're using.
3. They want to see if this feature is even popular before committing to supporting it for longer at all.