Most importantly, if they can crack and get the information they most likely can't use it unless there is an imminent and grave danger to the government's ability to stay in power. Because then using it would reveal their abilities.

Now there is a system that can be used for example. That system is once NSA identifies the list of suspects they forward that record to a another agency (FBI?) who then is in charge of finding other evidence, which could have plausibly be found anyway, and prosecute based on that. They can never release the initial reason why they got "interested" in someone as that might reveal the abilities of the NSA.