I am by no means a scientist, but I would think because the Moons gravitational field is considerable less than Earth’s it would take less effort to launch an object and with some guidance direct it towards Earth. Once Earth’s gravitational takes over such an object could obtain quite a bit of inertia and once again with guidance it may end up in an undesirable location and do quite a bit of damage, especially if it happens to have an explosive payload. But once again as stated earlier I am not a scientist, and no doubt there are many Freepers who could contradict this.
I am one ... the only quibbles I have with your post are:
1) It's a fairly complex calculation to hit a particular target on Earth, with a fairly narrow launch window.
2) I wouldn't bother with an explosive payload. Kinetic energy will be enough. Make your projectile dense and strong, so it doesn't break up on hitting atmosphere. You want it to hit ground.
Again ... this is easier said than done, but hardly impossible. The Moon colonists would have their own vulnerabilities, but they would also hold the high ground.
On another note, I do not dispute that "Brianna Wu" is a nutjob.