The NSA has [most likely] found a way to penetrate Apple/Google/MS/FB for specific targets i.e. they can get the info on any specific person/group covertly. The attack surface is just too large - TAO, Zero-Days, Insider threats, Financial threats, etc. The problem with that is things like needing "Parallel Construction [1]" to legally prosecute.
What the FBI is now doing is using a recent horrible tragedy to force SV companies to establish a precedent. Make no mistake, this has been long time coming. The Feds want to set a precedent both legal and 'cultural'. Ultimately Apple might cave, but the fact that they are raising a stink is very good news. Time for other heavyweights to join the chorus.
[1] http://thefreethoughtproject.com/parallel-construction-law-e...