As I have previously demonstrated, I'm no physics genius, so I can't explain EXACTLY why it doesn't come apart, but...

The propeller and engine mounts are designed to withstand these conditions and even a failure of a heating element on a prop blade would not cause the propeller to fail. They are designed to take some pretty heavy abuse (birds, ice, etc.) and not come apart.

Propellers are a very strong and a precisely engineered piece of machinery. It withstands some pretty incredible speeds and forces. They are pretty incredible when you really think about it.