Look at it like this - illithids enslave their thralls the old fashioned way, but any disloyalty is read right out of the slaves mind before it can blossom into anything, and even if a slave was pushed too far and snapped... attacked a master, all of a sudden the slave can't possibly fathom harming his beloved owner. And becomes passive.
And then later on, that feeling goes away. And the slave knows, just knows that if it happened again, the same thing would occur. They'd lose control of their own thoughts and feelings, and be unable to strike at their owner. There is no way to fight it. There is no escape. There is no hope.
Willpower just... gives out after a certain point.