They may be lying, or Bill Cosby, like many other famous people, presumed that he could do whatever he wants and get away with it because he is somehow "above the law" due to being famous.
There have been way more instances of famous people being beating the law because they are famous, than women claiming being raped by a famous person.
I am not saying its one or the other until the evidence presents itself. What I am saying is that I would not be surprised if the allegations were true.