why it is that in the MoRverse, being an occulamens makes you immune to it

Given what we know about how other magic in the setting works, my guess would be that it works partly by doing some mechanical Legilemency and keying off of intent to deceive, whereupon it triggers some sort of inhibitory condition. Seems in line with Eliezer's explanations for other apparently inexplicable effects, like the Comed-Tea.