In a nutshell, vestibular disorders are weird and the triggers are subtle. Certain movements won't bother most people, but if you smooth them out, adjust the speed, tweak the effect, things get weird.
I discovered, back when Doom came out in the 90s, that it didn't take much for games with a lot of motion - especially FPS - to induce motion sickness. I've sometimes had the same issue when I watch a movie in a theater.
But iOS 7 hasn't bugged me at all.
I am not intending to discount the experiences of the people reporting this, because I know how uncomfortable motion-induced nausea can be. But this has to be a very tiny percentage of iOS users.