Short of fishing golf courses and the like, the No Fishing signs are many times posted to reduce liability to whomever owns the land the body of water is sitting on. I fish an area locally that also has No fishing signs posted but after talking with the management on the matter, it's to prevent them from being liable for any/all accidents that may occur on or around that particular body of water. It probably depends on the specific area. My two cents worth. Want change?