Ignoring the fact that this probably the first pyramid scheme in that world so they wouldn't already be aware of it, Pyramid schemes have always been effective scams because they seem like a good plan until it's too late to back out.
They target people who are already down on their luck and...
You can't be brainwashed if you don't have a brain in the first place.
Theoretically, if he was the demon lord and had brainwashing powers, he could be trying to keep them alive to enslave them.
Thank for the confirmation. I read the WN a couple years ago when this went on hiatus. Between the hiatus and the slow progress since it came back, it's hard to tell where we are in the story sometimes.