Well, because they expect you to FF it. So they position themselves to grab you out of it. Then when you get grabbed, Marth screams. If you don't FF, often they'll try to grab and they'll get hit by the very last few frames of the nair, and it knocks them over. If you know they aren't going to run into it, then FF it, but otherwise fastfall isn't necessary for spacing, because most people think they can sneak an attack in before you hit the ground. Not true. Once your opponent wises up to it, you have to start dashing away as soon as you hit the ground though. The good thing is you can decide last second whether you want to fastfall, so you can choose which one you want.
M2K often does nair -> immediate dash away. Then he pivots the grab.