Even putting the engineers in the position to have to make a choice to resist means that you damage their career prospects whichever way they choose. To be known as 'that guy' who {supported a corrupt government/supported terrorism} polarizes future job choices. There's no real upside. As someone asked to do this, I'd be asking for all future earnings up front from the FBI, so the economic worth of this is really 8 or 9 figures.
If you are one of the very few encryption engineers at apple that can do this, you are already one of their highest paid employees, and also one of the most sought after in the entire realm of tech. Quitting over this is likely to drive your value up even higher.
The FBI when it compels people to do things, pays nothing to have them do it. That's the power of the law.
