First off, good on them and I hope they get more senators to join their filibuster. I find it unbelievable that any politician can actively support unconstitutional spying programs like this and stay in office.
That said -- really, out of that list, at least Patrick Leahy and Rand Paul are really libertarians, they ran as republican and democrat to get into office (successfully, they've both been there for years.) The US has a broken political system, with a near-one-party system. Since these parties combine people that should (in a working political system) be members of probably half a dozen parties, you can or course find some reps and dems with polar opposite political views, and you will find "far left" and "far right" individuals in them. But you look at party line, both parties to avoid "alienating their base" have nearly the same political platform really -- by British standards, the reps are center-right but very near center, the dems are center-left but very near center.
Of course, both blame the other party for all problems. For example, both parties lie and claim they want to fix the federal budget problems, which are entirely the other parties fault. But, in fact both parties actually for increased gov't spending and tax cuts.
I have no idea why both parties have whole handedly decided they should ignore the Constitution and civil rights. But, that's the rub -- I can care about my rights as much as I want, without any 3rd parties it's highly likely as elections come up the choice I have will be two people who don't give a damn about rights.
The big two issues causing this -- polls, and debates.
Polls -- personally I've been polled twice -- the first time, the pollster asked which candidate I was voting for, then named the democrat or republican, and they hung up when I said I was voting 3rd party. The second gave a choice of "press 1" for some republican, "press 2" for some democrat, "press 9" for someone else. When I hit 9 it said the choice was invalid and hung up. It's kind of a vicious cycle when the media uses polls that usually don't even had 3rd parties as a choice, they therefore don't mention or include 3rd parties, so it won't even occur to most of the public to look into them then.
Debates -- just 1 example, there was a big row during a few of the early presidential debates this last time around, because they excluded some 3rd party candidates due to low poll figures, but had probably half a dozen main-party candidates for each main party, several of which had actually polled lower than the excluded 3rd party candidates.