That's not the same scenario. Your scenario involves listening to everyone. The GP's scenario involves listening to a specific person with probable cause.
Given that the police wanted information after having already received the threats, they had to have requested some combination of historical and future data. They almost certainly wanted historical data primarily so that they could find the person who sent the threats.
There's no way to satisfy requests for historical data without capturing data from everyone.
They don't need to. They only need to turn over data they already have for that user and potentially any future data that is generated for that user. Wiretaps work the same way.
I hate this argument. So there was no crime before email? If you have pc, go get a wire tap or install microphones in their house or get an informant to infiltrate. Police work is hard and requires work but it is also what grants us presumption of innocence.
Patriot act which supposed to protect population from terrorists not only failed to accomplish it, it was used in war on drugs.