I might not be the sharpest tool in the shed, but doesn't it stand to reason that if you were standing further away from the hitter, wouldn't it be more difficult to properly see the swing. After all, wouldn't the tosser/pitcher be trying to avoid being hit by the batted ball (we know there is some sort of screen being used)?
Have you ever tried tossing or pitching from behind a screen from some far distance (more than 15 feet), get it over the plate, and still be able to evaluate the hitters swing. I DON'T THINK SO!!!
If you can do this, let me know, because I would love to see this. I will even be more than happy to have my son take a lesson from you.