Laws exist in many areas of the U.S., which govern swimming pool fences and their installation. Whether or not they are mandatory has little bearing on how deadly a swimming pool can be to a child if she falls in and no one is aware of it. Supervision is not 100% foolproof, and yes, accidents do happen.
That is all very easy to read when you have not lost your own child to drowning. Visualize the tragedies that can and do happen every single day in the United States, (a staggering 85% of child drownings occur in the pool of a friend or relative) and you will soon realize how vital a swimming pool fence is.