people love reading things like that. it seems taboo, and starange. which is the main reason it intrests them. because it is something outside of what they have been tought. for example, a mother tells her teenager son that he is not allowed to smoke pot and do drugs. but yet, he still hits his pipe every day.
or a mother, teaches her daughter the reasons not to have sex. but yet that just encourages the girl to try at. she likes it, and does it regularly with many different ppl.
the more you tell someone not to do something, or that it is wrong, the more they desire it, the more they want to know, and the more they do it....
and incest is much like that. most are told it is wrong. but yet, it still intrests them. and they want to know more about it, and act on it.
if the world said "ok incest is legal, fuck ur family members!!" then i highly doubt many people would even care to try it.