I'm not usually too fond of engaging in political discource, but here goes...

Replacing legislators is indeed part of our system, yes; but who they're replaced by isn't a matter of choice for the American populace, to an extent. Yes, we get to vote for our representatives and various other government officials, but what we don't do is determine who's running for those positions. There are barriers to entry for lawmakers, and those barriers filter out much of the populace. By the time votes are being cast, the eligible pool is down to a comparatively tiny group of people; if nobody in that group feels the way you do about the issues important to you, who do you vote for? Nevermind the ability for them to change their stance on something once elected, for one reason or another (for example, Hilary Clinton's push for government-funded healthcare, which mysteriously stopped after she was given several millions of dollars by pharmaceutical companies).

Beyond that, the Constitution is not an absolute defense of our rights. Yes, as it's written, it should be. But the Constitution is merely a sheet of parchment with fancy writing on it; as the document itself states, it's up to the populace to elect officials to carry out the will of the Constitution. If the people we elect don't care to do that, then it has no power. Officials can merely ignore the Constitution if they'd like; who will punish them? We do indeed have a system of checks and balances on power, but that operates with the idea that those checks and balances will be enforced. If nobody's willing to do so, what good are they?

As a recent example, the Defense of Marriage Act was overturned. This act was blatantly unconstitutional; it violated the Full Faith and Credit Clause of the Constitution, which states that legal actions in one state are applicable to all states. Ergo, if you commit murder in one state, you're wanted for it everywhere, not just where you committed murder. Conversely, if you're married in one state, you need not apply for a marriage license in other states if you move; it's carried with you. However, DOMA stated that same-sex marriage is only applicable to the state the license was granted in. By the writing of the Constitution, that shouldn't have been possible; defying a central clause of the Constitution (and, truly, one that's necessary for law to have any meaning at all) shouldn't have happened. Unfortunately, nobody wanted to act against it, and it was put into law. From there, it took more than a decade to get it removed. If a law can violate the Constitution so fundamentally, yet exist for multiple presidential terms, it's clear that the protection on our rights only exists as much as lawmakers wish for them to exist.

Whew. And with all that, I'll climb down off my soapbox, I suppose. I agree that the NSA's conduct and invasion of privacy are quite quesionable, but as long as we're only given candidates that are compliant with them in our polls, there isn't much we can do to reverse the situation, aside from hoping someone more resilient comes along for the next election.