A play for political power. The NSA probably doesn't care if data is encrypted---they most likely already have a back-door to take data before its encrypted or after its decrypted. So if encryption is used, the FBI will be dependent on the NSA.
Snowden leaks confirmed that hypothesis. Probably still true for economic reasons as other commenter noted.
We know from Snowden that they do not have backdoors into everything.
Its about economics, if they can listen in on the exchange its essentially free. If they have to backdoor every end user device, its gone cost a literal shit ton of money.
It's really pretty damn easy.
http://tech.slashdot.org/comments.pl?sid=8623643&cid=5131356...
Win 7, 8 and 10 are done. OSX has spotlight upload to microsoft in plain sight. What else does OSX have under the hood? Phones are pretty much a foregone conclusion in my estimation, with the exception of cyanogen mod or something.