The key word in your reply was "fool". Anyone approaching a worker on a mower should know to approach with eye-to-eye contact. My wife knows better, I have neighbors on both sides without kids(and they know better). I hope you know that you weren't responsible for that lady getting hit with a rock. Besides, I do keep my eyes open when mowing(and the likelihood of "hearing" someone approach is slim and none:0)
When I am mowing I try and keep I eye on the whole yard it is not easy when I am cutting 4 foot tall weeds and looking just in front and to the side of my next pass.
You would be surprised at what I find in yards. Worst thing is axe heads.
When my wife is using the trimmer we watch out for each other. I never make a pass with the shoot towards her.
I have had thing cone out from under the mower that would have killed her.
I hit a rock 2 years ago. It hit the door on my truck and stopped at the other door.