While MWD's and Police K-9's are in fact tools, they have a very important and vital function to perform. Every handler that I know treats their dog very well, cares for their dogs and love their dogs. With out that bond the working relationship is surely impacted. Since I did not see what happened, I'll take your word that the dog in your opinion was strongly corrected. If there are 4 male working dogs living at that house, perhaps the GSD became aggressive or started to instigate aright with one of the Mals. Perhaps the GSD became aggressive to the handler. Yes, shocking as it may sound….some working dogs are handler aggressive. Perhaps the handler was trying to prevent a fight between the GSD and one of the other dogs. That is one way to take control very quickly and assert dominance over an aggressive dog, but must be done quickly and carefully.
From the scenario as you describe it, my thought would be that the GSD probably went for or was about to go for one of the other male dogs. I have been in this situation myself, owning 3 working dogs at one time. A dog fight between two male working dogs is not something to be taken lightly, even if one or both dogs are retired. Fights like this rarely end with out serious injury to one or both dogs and the handlers. Picking up the offending dog quickly changes his state and placing him firmly on the ground will usually end the situation. One must be fast, strong and not afraid to instantly intervene when in this situation. When done correctly, instantly and with significant power the fight is taken out the dog, the dog is not injured and the handler is not bitten. It is a very effective way to gain control and stop a dog fight by a strong handler.
I am not recommending this technique to novice handlers or for owners with "softer" dogs. Having owned some very dog aggressive dogs, handler aggressive dogs and sufficiently hard, strong dogs over the years I have used a similar technique on occasion. With that said, I do not like "alpha rolling" dogs nor do I believe in doing that. But when out in a yard with out a leash, sometimes you have to take charge the best way you can as quickly as you can.
It sounds like you love dogs, as do I. But if you have never handled a working dog you would think this was severe or harsh. If I am correct in my thinking this was not very severe at all.