Everything has a reason to want to go somewhere - usually it might be to attack a PC, but maybe it's giving them a vision that the PC is over there. And extra juicy. And vulnerable. Or maybe it's something to mate with. Or that one color of prismatic puce they can't resist.
Remember that by core rules (Player's Handbook, early on in the power section) the flavor text is purely flavor text and may be completely replaced at the whim of the player with anything else appropriate. Doing so is not only a good idea in some cases, but can make the game more fun.
Still lame.
The default rule is that affected foes KNOWS what is happening to them. So, how come the foe does not know that it is affected by an illusion?
In a combat, nobody should want to stay "rooted to one spot". And once the foe knows that he was affected by an illusion (by making a save for example), he shouldn't be "fooled again" in the same combat by the same effect from the same spell.
But, the foe is "not fooled" with 4E illusions. They are not really illusions. They are real effects. They are real conditions. It's not illusory in any way, shape or form.
Illusions are all game mechanics now and no fluff. You don't fake out foes with them, you just throw a condition on them. The entire "the PC decides what the illusion is and the DM decides if it works" (even using rolls to do this) is gone. It's now just a condition like any other magic.
Yes, one can handwave it away and say (like you did) the foe sees something in its mind that causes it to act that way, but that is bogus too. In a similar situation, the same creature could act differently. Instead of moving towards a spot, it might want to move away for some reason. Exact same stimulus. But, that doesn't happen because illusions are just game mechanics now, no different than any other Slide or Push or whatever.
Cause Fear is how mental illusions should work. The NPC decides where it goes based on the NPC (i.e. DM decides based on the logic of how the NPC works), not based on the spell. The NPC could run away to the right or away to the left or straight away with Cause Fear. That doesn't happen with illusions.
Whatever. They gave it the name Illusions, but it's anything but that.