This can sustain itself in part because there is a relatively clear divide between the mainstream and alternative comedy circuit. Within the mainstream circuit

This can sustain itself in part because there is a relatively clear divide between the mainstream and alternative comedy circuit. Within the mainstream circuit there is much more of a culture of sharing material, so one can be of the opinion that joke theft is just something that happens on the mainstream circuit. It doesn't majorly impact the victims on the alternative circuit because the audiences are different, and the concept of ownership of material within the alternative circuit is a lot stronger and so the people who matter will still attribute it to you. I don't know enough about magic to know if there is or isn't a similar divide.