You ask, in effect, if the administration already had the ability under its war powers to detain people, why create a new bill that re-affirms that power? The answer is political. The conservative GOP wanted to re-affirm those war powers that benefited the CIA, and at the same time some (but not all) liberal elements of the Democratic party wanted to clarify and rein in those powers. There was a fight, and then a compromise bill, which, reading between the lines, attempts to punt the thorniest parts of the debate to the Supreme Court while creating an immediate statutory authority for CIA and DOD to continue doing what they're already doing.
Yes, this is a clusterfuck.
The problem in the US is that we were hit with one of the most damaging, best organized terrorist attacks by a non-state actor during the least competent administration of the past 50 years. Congress panicked and granted the Bush administration a wish list of military powers, and we're spending the ensuing decades gradually cleaning up after the resulting mess.
That's a problem. But it's not a sign that the US is gradually turning into a dystopian hellscape.
I don't care, except for two things. First, when people bring up the NDAA, they set off a "someone's wrong on the Internet" sensor that, as a nerd, I am unable to suppress. Second, as much as Internet psuedo-activists do not want to hear this, the "NDAA" issue is in fact a corner case in US policy. Indefinite detention of political dissidents in China is not a corner case. It is a daily fact of life, as it is elsewhere in the world.
You can make millions of dollars spinning conspiracy theories about the evils of the government and big corporations in America, and the reality is, nobody really cares. Try doing that in Russia; see how long it takes for you or your coworkers to end up dead.