Because there is no way to explain it.
Your words, not mine. I suggested magnets. Maybe we weren't watching the same video? 
There is no way to explain it WITHIN THE FLAT EARTH HYPOTHESIS.
Magnets? Uh okay, show me something even close to that that uses magnets. Maybe we were watching different videos. The one I saw showed 2 1/2 minutes of a very large tricked out camera seemingly floating in mid-air AND a person doing the same thing. I've never seen a trick like that done on earth using magnets.
The only way to explain this is the reasonable explanation given to us, and that is they are in orbit. When the station begins to accelerate, everything that is not tied down begins to move in the same direction. That's not even something you could do in an aeroplane flying parabolas (That the footage is so long *and* so stable disproves that explanation anyways).
Come up with whatever explanation you want, but you have to back it up with evidence. You might as well suggest they used magic faerie dust.
There are many many ways to do this. There is the vomit comet parabolic flight airplane (because NASA makes the best airplanes) that could easily fit the room he was in.
You're right, gosh why didn't I think of that, I really should have addressed that. OH WAIT I DID. Seriously, did you even read what I wrote?
Find me Vomit Comet footage that is:
1. As long as the footage we see from this ISS video.
2. is as stable as the footage we see from this ISS video.
3. accelerates in the middle of the footage without falling out of the parabolic arc.
Sorry, the Vomit Comet simply can't fly in parabolic arcs for even half that long before it has to pull up (at which point the "gravity" goes from 0 G to 2 G and everything that isn't tied down hits the floor hard), nor can the Vomit Comet fly in such a perfect parabolic arc that the footage is so stable, especially when it has to accelerate in the middle of the manouvre to simulate the effect of accelerating in orbit.
Try a little harder next time, you're back to square one and stuck with faerie dust as the only reasonable explanation (other than the real answer: They are in orbit)
Hey at least you aren't dismissing it as CGI and citing a terrible movie like Space Cowboys as the ultimate example of faked zero g footage.