The working suspicion is that someone who has immediate and private access to these horrible sites, is liable to be suspected as possible having visited these forbidden sites.
First of all, as others have said, even if someone has visited these sites, one is obligated to assume that he has done ?????.
Additionally, with regard to being a witness, we don’t really care what he does in private, as long as he hasn’t been doing shady things for the sake of money. Check out the ???”? mentioned above, and the ????? in ??????? of ???? ???? ?????? – every disqualification mentioned (other than ????) is regarding monetary suspicion, where we are worried that a witness may have taken money to testify.