They can be made fun of because “rednecks” and “hill billies” are primarily white. Just like I can be called a white-boy when playing basketball. Would I ever dare to say “black-boy”? Heck no! I would get murdered. This is even though I have black friends.
Someone who calls me whiteboy is gonna get called whatever demeaning term that is applicable to their race.
They do it because you put up with it.
P.S. I’m armed 24-7 and will not be murdered without a firefight.