This was probably the best possible case for them to try this with. They had a terrorist attack on American soil, and one involving Islamic extremists at that.

This was probably the best possible case for them to try this with. They had a terrorist attack on American soil, and one involving Islamic extremists at that. Not only is there less than zero sympathy for the attackers, there's also an element of fear that you have here as well to go along with the hatred people tend to feel towards terrorists and mass shooters. They probably thought that if it came down to a PR battle, assuming Apple didn't just roll over for fear of the PR consequences of "obstructing a terror investigation," they had it in the bag. Turns out? Not so much.