The international legal framework of sovereignty basically says you are owned. (Not universally de jure, but pretty much de facto.) Whatever rights you have are effectively granted to you by your country. Unfortunately, this notion is seldom given any thought, and the current most visible proponents of such an idea are unpleasant angry underclass men using it as an excuse to behave badly. There are others who have given thought to this, however, and it is part of the motivation behind such things as The Universal Declaration of Human Rights.
https://en.wikipedia.org/wiki/Universal_Declaration_of_Human...
In my experience, law enforcement does not make their own jobs harder on purpose. If there is an easy way to get that data, they would use that way to get it.
So you don't want any hardware to have access to main memory if it doesn't need to. For instance, you can use an IOMMU to ensure that devices can only access the specific areas the OS wants to allow them to DMA to/from, not all of memory.
The phone isn't always locked and encrypted; for example, whenever the user is using the phone it's unlocked and decrypted.
I'd guess "security by obscurity". Just because they have the device rooted via SIM card doesn't mean they have available a signed build of a multi-gigabyte OS with most security libraries expunged.
I want to disclaim that this is pure speculation. I have no insider knowledge or indeed any particular familiarity with the institutions in question.
The FBI may want this authority and this precedent and think that this is a good chance to get it. They may say, "Well, the San Bernadino case is a high-profile case that may sway people, including judges, who would otherwise be less inclined to back our request. Who knows when the next nationally-publicized case will be in which the likely perpetrator carries an iPhone?" They may also believe that the current political climate is good for their case.
And they probably also believe that there's no harm in trying. If the courts rule against them, they haven't lost anything. If the courts rule for them, they get a brand new tool.
The iphone contains a sim card.
A sim card is a complete, general purpose computer with its own CPU and RAM and the ability to run arbitrary java programs that can be uploaded, without your knowledge by your carrier.
You are owned. Deeply, profoundly, in ways that you have no way to manage/mitigate.
The real question, for me, is why authorities are dealing with Apple at all and not just working with the carriers who have proven to be their trusted allies.