I would think we are responsible one way or the other. The beach is public and ,at least from a legal point of view, we are excercising an activity that has some potential of danger to someone walking by, even if they see you. On the other hand if they are warned that the back cast will come and ignor the warning that may be a different story. Also, someone standing behind to watch who is warned is also... but I would think that if they don't move you have to... which is really no big deal since moving ten feet to one side or another should not matter depending on your direction of cast and where you want to get to. Here's the real issue. If you don't see them and they don't see you or they don't know about the back cast, all things being equal the law might presume that since the nature of the activity can be dangerious, the location, and the use of it by the public, you would ,or should have forseen that without checking behind you you would be liable for any injury you might cause. Just my opinion.