Only because there are so many non-incestuous couples now. Say for the sake of argument the odds of a non-incestuous couple having a child that has genetic complications is 1 in 200. How many such couples are there in society? Thousands? Tens of thousands? Hundreds of thousands? Millions? Those genetic complications are going to occur a lot. The chances for a non-incestuous couple of having deformed offspring are much lower than for an incestuous couple. Indeed, the chances of someone born of incest having children with fatal genetic failings (even if they have sex with someone not born of incest) are also greater. The same is true for those born as a result of cousin-cousin relations.
Here's a brief summary:
http://www.geneticseducation.nhs.uk/genetic-glossary/249-cousin-marriage
That's a question I'm really not comfortable with answering - mostly because I'm not entirely sure which way I'd answer. But also because of what I might learn about myself if I
do answer it one way or the other. If society starts sterilising people (as a method of preventing them from having kids they could pass their condition on to) because they have 'undesirable conditions' then where will that end? Like I said before, it ends up sounding really eugenic-y.