I do not believe that a shock collar is cruel.
That is a viable method for keeping the dog safe in a large open area. The dog will learn the boundaries quickly.
What is cruel is what the owner of these dogs in the story allowed to happen.
When I was much younger, my step uncle in south carolina had this wonderful husky named bear. I used to love to go up there and visit bear. They let him run free of course. Up there there was about a mile in between each house.
One summer I went up there hoping to see bear, but bear had been shot one day and died. I was very sad.
We borrowed the one we have from my BIL, I am going to buy her one, just in case she forgets.