How do you circumcise a hillbilly? Kick his sister in the jaw.
Why joke specifically about hillybillies?
There is no explanation to second class citizens.
Joking about hillbillies commonly participating in incest propagates a negative stereotype about them. It causes them to be viewed in a negative and disgusting light which creates a bias against them. Try be more understanding of those in that situation and if you'd like to be referred to as incestous.