Why are incest jokes blamed on Alabama online?

Depends upon where you are. In Oklahoma we used to joke about "Arkansas, the state of 2 million people and 3 last names". I have heard quite a few about West Virginia as well.

{@alabama] who ever is posting the cruel jokes should keep them in the family

West Virginia, also---- The Alabama thing I think comes from the movie Deliverance. Many people in Virginia think that movie was in West Virginia.

Any place thats really country such as Alabama and the Ozarks makes people seem slow because they speak slowly which is a possible indication of being inbred according to Northern sterotypes.
Same goes for not wearing shoes, chewing on hay and all that other cartoon stuff but there is a bit of truth to it.
More cousins continued to marry one another in rural areas long after it wasn't accepted in the cities.
Look at the book Gone With The Wind. The main guy Ashley was married to his cousin and it wasn't even part of the storyline. That used to be very common.