The following articles clearly explain the side-effects, "fall out" and dangers associated with physical punishment and threat based forms of training and the training equipment that uses pain or discomfort to get compliance. While punishment based training works, it takes a toll on the mental state of every learner subjected to it. Sometimes seriously enough that the animal must be killed.
Because there are ways to train ANY behavior and ANY animal without using threat or physical punishment, what would cause a person to choose to use pain (or threat of pain) over a training system that is much more pleasant for the learner and can achieve the same, or better, level of compliance and precision? Mostly it is because the trainer has not learned to use the reward based training methods or has not used them correctly. To find out why you should want to make the effort to learn the more pleasant training methods, read these articles:
About Dog Training Classes That Use Punishment:
"Dog training methods: their use, effectiveness and interaction with behavior and welfare"
Full article: http://www.antrozoologisenteret.no/artikler/art_training_methods.pdf
Authors: Hiby, E.F.; Rooney, N.J.; Bradshaw, J.W.S.
Source: Animal Welfare, Volume 13, Number 1, February 2004 , pp. 63-69(7)
Publisher: Universities Federation for Animal Welfare
A better way to train:
If you want to know why most reward based trainers never go back to threat/punishment based methods of training read "You Can Cross Over, But You Can't Cross Back" by Donna Dufford: