See, I like to give credit to writers who actually try to justify why these ridiculous things happen, but I'm not going to give STID's writing trio credit for stuff they didn't even bother to put into the movie. Speculate all you want, but if there's nothing in the film that confirms your speculations, I'm going to conclude that they didn't put much thought into it.
By that standard every Star Trek movie needs to be seven hours long and every episode three.
I'm just wondering when some Star Trek fans started needing everything spoon fed to them?
If Marcus actually had that much hidden influence to stop anyone from interfering with his secret business even when he's making a show if it in front of Earth, why does he need to worry about anything at all? Why not let Kirk bring Khan back to Earth and tell his followers to kill them the moment they arrive on Earth? Why is he in such a hurry to destroy the Enterprise when he can sit back and let his loyal officers deal with Kirk and Khan?
Because once you get back to Earth, you actually have other factors to deal with like the Earth government, the media and so on. By destroying the Enterprise before she ever makes it to Earth, he can control the flow of information. He was going to destroy them near the Neutral Zone, but they took off before that could happen.