But why is it always nazis?
Nobody jokingly organizes a movement to eat the landlords that actually causes some landlords to get eaten, but the same cannot be said about "jokingly" organizing a hate mob that actually hurts people.
Also a product of someone taking 4Chan seriously. There used to be a saying every time 4Chan was mention back in the day. Something like if you have a community where you pretend to be fools, an actual fools would think they've found their company.