I think all the people that make those simulate. I read at on point, bskprs? or another mod maker saying they plop down an invisible spider and zombie and then remove them to come up with calculations to draw the grid. MC itself doesn't play well with artificial light sources either so that causes issues as well. I do wish the magnum torch / chandelier gave out some sort of grid while holding a special tool a la the precision hammer from MFR. that would be great.yeah it is.
I suspect that to save calculation time NEI detects light levels rather than seeing if an actual mob can spawn. Integrating the magnum torch specifically may not be a huge challenge for chickenbones, but it would have to be special code for that specific block most likely, which I suspect is why it's not been done - there's no common system for preventing mob spawns besides light levels, and force-checking would be sloooow.