In the clip, the lifeguard that pulled the girl up said that the difficulty with that particular pool is that it's very deep and when there is a lot of activity they simply can't see if someone below the surface is having difficulty. It specifically goes off when someone is in contact with the bottom of the pool for 10 seconds.
It's more of a fire alarm. You wouldn't say, "Hey...let's not bother to blow out those candles...the fire alarm will let us know if something's wrong, won't it?" (OK...maybe there are people out there who are that stupid.)