Here's why. In the original "Civil War" story arc in Marvel comics the flashpoint of the Super Human Registration Act which curtails the vigilante activities of superhumans was the flagrantly irresponsible acts of the New Warriors (my favorite 90's superhero team). The New Warriors flamboyantly jump a small group of D-list supervillains who are basically just hanging out in their own backyard, while a TV crew films the Warriors as part of a reality program. The teen heroes are reckless, arrogant, and are unprepared for the predictable response of the villain Nitro (who admittedly was juicing on Mutant Growth Hormone, making him even more powerful). All of this was against a thirty-year history of the Avengers having a complex relationship with the US government, the United Nations, and SHIELD, best outlined during the "Peter Gyrich" era.
So it makes some sense, a semi-natural evolution of the Marvel Universe (the "semi" being cleaned up by having it revealed that the Skrulls have been stoking the fires for a long time).
In the Civil War movie, the impetus behind the attempt to put the Avengers under the control of the US government with the blessing of the United Nations seems less tenable. In the dramatic scene where Ross rattles off this litany of venues that have been devastated by superhuman conflict, I kept thinking to myself, "that wasn't their fault." New York? That was an alien invasion. That's like blaming the US Navy for shooting up Pearl Harbor. Washington DC? That was SHIELD, and would have been a lot worse if it wasn't for Cap & Co. The only real argument is Solokovia-or-however-it-is-spelled because that was Ultron, who was created by Tony Stark, whom I can not figure out is not under criminal prosecution for creating and unleashing what is for all intents and purposes a malicious computer virus and mobile weapon platform. And hey, we already have laws that are about people doing that. Why do so many people, including several superheroes, engage in the intellectual disconnect of blaming the people who curtailed the damage?
And you know which one really doesn't make any sense to me? The Vision. At this point I'm convinced that the only reason he could lift Thor's hammer in Avengers 2 is because he's an inanimate object, and that the hammer obeys the laws of physics (sort of) when it comes to being affected by inanimate objects, and only drops anchor when a person tries to move it. Because here's the thing: I'm pretty sure lying to people and detaining them against their will is a no-hammer-lifting-for-you kind of activity. Because if Cap can't lift it (and he can in the comic books), then the "hey little lady you got to stay prisoner here while I try to get my android lovin' on" Vision sure as hell can't.
So the premise is thin (let's not even get into the whole thing with Bucky Barnes. He was brainwashed. It's established in The Winter Soldier. That means capture if you can, because he's innocent of wrong-doing.) There's no real dramatic tension about who is right and who is wrong in this, like their was in the comic book series. In the movie it is the emotionally-damaged Iron Man, the ridiculously passive War Machine, the confusingly naive Vision, the vengeance-obsessed Black Panther, the underage and easily-manipulated Spider-Man, and the Black Widow. The other side is clearly the heroes. In the comic book series you saw the gradual progression of the outcomes from the Registration Act, culminating in the death of Goliath. The "casualty" of the MCU movie has nowhere near the same emotional gut-punch.
All of which is my saying that while Civil War is a good movie, and even a strong offering in comparison to the other MCU movies out there, and leagues better (pun intended) than what DC is putting out, it deserves some critical attention when it comes to its breezy and at times confusing plot.