I'm confused what the problem is. Is it since the general population as well as terrorists use iPhones, NSA shouldn't try to hack into it? If so, that seems rather naive of real life.
Over the past several years I've seen various news articles of criminals hacking into phones. Is it OK for criminals but not those trying to catch them?
I know what you are trying to say, but your argument falls apart completely if taken to its conclusion. You are asking why it is not OK for law enforcement to break the very laws that they are trying to enforce in order to catch others who are also breaking them. In general it is obviously not OK to do that, and it's not OK for the criminals to hack into phones either - precisely because it is illegal.