Well, I still think this is good enough, because:
1. If they eat all the humans, they die.
2. If they defeat the shrine maiden, the border crumbles and they disappear because of Common Sense.
3. If they eat the humans, but the shrine maiden, she has enough power to kill off all the youkai, if not, see point 2.
So maybe it was human enforced, yes, but it was made to make a peaceful life for both sides. It may seem unfair, but if that's needed for peace and survival, then that's it. And they were made because the youkai became too powerful to contain, so it was made because of the survival of the humans.