Hamilton says, "So if we open up this doorway...then theoretically they should be pulled back in."
The implication is that anything that has been within the Phantom Zone would feel a pull back towards it. This includes the Black Zero and all of the Phantom Zone criminals who have been in the Phantom Zone by virtue of their original imprisonment and use of the Phantom Drive to come to Earth. Additionally, Superman has spent time in the Phantom Zone having used a Phantom Drive to come to Earth. Lastly, it is implied that the World Engine is empowered by Phantom Drive energies since they link the two and we have the lines: "Bring the phantom drive online. / We are now slave to the world engine." (remember that the Phantom Drive is a novel technology developed by Jor-El that didn't exist at the time of the World Engine's inception)
Thus anything struck by the gravity beam would have arguably been subjected to Phantom Zone energy as well (such as the surrounding debris and Superman, having been below the beam under the World Engine).
This explains how a number of things were drawn in and towards the singularity, but why Lois Lane was able to fall freely away from the singularity. She arrived on site via the C-17, so she had never been in the Phantom Zone or hit by its energy via the gravity beam. It explains why Superman, however, has to resist the pull after catching her.
Superman's ability to resist the singularity proves that it its pull can be overcome. In his case, likely by strength, but in Zod's case, likely by range. Whatever remnants of the World Engine remained, similarly, did not find themselves pulled through the Earth towards the singularity. Thus there was a limit to its pull.
Incidentally, the Scout Ship arrived to Earth without the benefit of a Phantom Drive, nor was it in the gravity beam, so it too would have resisted the singularity's pull if it had been in range.