To a large degree, data storage is an extension of what a person knows. Why bother memorizing a phone number when you have hardware to do it? Why bother memorizing a hundred passwords when you have hardware to do it? Even our music collection is on hardware purely because our ability to memorize it is imperfect.
The moment a law is passed that mandates law-enforcement access to our electronic devices, we are giving them access to what we know. Today that may or may not be reasonable. But tomorrow, the day after, or a hundred years from now we will have these devices integral to ourselves. Implants within us, most likely, that augment our memories. It's not unreasonable to predict a (likely distant) future where a device taps our optic nerve and provides us "augmented reality". Can't remember the name of the person you're looking at? The device will do that for you. But it will also be able to record what you see, or hear, for future perfect recall.
So what happens when the iPhone law is applied to internal storage? It's mind-reading. This legislation is one step shy of "police must be allowed to read your mind if it is possible". That disturbs me.