The letter, signed by Apple's CEO Tim Cook and published Tuesday, warns that complying with the order would entail building "a backdoor to the iPhone," creating "something we consider too dangerous to create."
Here's my take: The "backdoor" already exist on Apple devices, but Apple is publicly alluding that such doesn't already exist so to appease its current and future customer base. Brilliant!
There are already backdoors in Apple and Microsoft products. Look it up. If you want privacy and security, get and use Linux. Many kinds are very user-friendly. My grandmom uses Linux Mint, which has the look and feel of Windows 7, but is much more secure and doesn't have MS backdoors.
Regarding the backdoor on Apple devices and Windows OS, it is very dangerous because criminals/foreign secret services can get easy access to these devices as well should the access keys be stolen/leaked. Actually if you can prove it already exist, you can sue Apple or Microsoft for a lot of money succesfuly world wide imo.