What gives the impression that voters have anything to do with how the NSA operates? Sure, in theory one could try to vote someone into a position of authority who could influence their operation, but how realistic is that? The people that head up the NSA are 'technically' appointed and confirmed by elected officials, but an administration is not going to appoint someone who isn't on their side, so congress has limited options. In the end, it becomes a choice between (ultimately) identical candidates. Administrations may boast they will clean things up when elected but once they win and realize what they have, does anyone really think they are going to throw it away?
Many campaign promises get broken... those that have to do with reducing one's own power almost always get broken.
In the end, intelligence agencies and law enforcement are usually very focused on finding ways around any limitations to how they believe they can be most effective. That does include finding ways to circumvent legal barriers.
Sadly, it seems to be largely human nature... how many people can resist snooping at a diary when found, or a colleague's pay check found in a drawer while looking for a pencil, etc... This is simply on a much larger scale. Yes, it is very wrong, but if you think someone, when voted into the coveted position of being able to know most secrets, is going to give up the advantages it offers.... think again.