It is commonly acknowledged that there is no encryption that the NSA can not crack. However, when faced with strong encryption, you have to know whether it's worth the effort or not. The NSA's datamining operation is based on trawling everything, everywhere, in order to see if it's interesting, and strong encryption defeats that.

So, the need for a backdoor. The theory is "OK you can have strong encryption, but the NSA gets to look at your content".

_{It is rightly acknowledged that people of faith have no monopoly of virtue - Queen Elizabeth II}