Why the MiB can't cross the ocean and hasn't killed everyone (yet) by Cedestra

First theory. Thank you for reading and discussing.

I reviewed the episode The Constant and noted that Daniel explains to Jack and Juliet that anyone who's had a high exposure to radiation and/or electro-magnetism experiences side effects when travelling over the threshold to the island.

If Desmond was having such a hard time travelling across the boundary to the Island and the rest of the world, as a human being, imagine what a hard time someone/thing would have if they were made almost completely of electro-magnetism. Like, perhaps, the Man in Black.
I betchya that when he tries to cross the line, he's going to be incapacitated. This would be why he's chosen to make nice with a solid group of followers. He's been honest with them, leveling with them about a lot and being, in all appearances, honest and caring. He's smart- he knows that when he'll need people to cross over with him and that his "friends" are much more likely to not destroy him in his moment of weakness than his subordinates would have been.
Little side thoughts to go along with this theory: I assume that the boundary around the island is some form of electro-magnetic forcefield. The fields set up by the Dharma Initiative are assumed to be EM fields as well. I'm guessing EM waves cannot cross EM waves. I also theorize there must have been some weak but strong enough material in the ash used to keep the MiB out of the temple, like crushed lodestone or something else magnetic.