i mentioned the forced retrieve in my post which was from an american video.
why does the retrieve have to be forced?
and why should a dog retrieve out of fear of pain?

The retrieve does not have to be done via compulsion. See Ed's video on Bernhard flinks motivational retrieve.

But, also understand that compulsion for any reason need not be a dog screaching in pain, not understanding what is expected of it. The force retrieve is a staple of many many police dog programs, such as KNPV, it is used effectively by field dog trainers over and over without detriment to the dog's attitude towards the work. I use a retrieve in my tracking work for evidence (an experiment I did with this dog). It is a compulsive retrieve and the dog is a motivated successful street tracking dog. I also used the forced retrieve in schh and WPO competitions and used compulsion to calm the grip on the dumbell and subsequently to the evidence in the track and evidence search.

I don't recommend forced retrieve for every police dog handler or police dog. I also don't use the compulsive retrieve to substitute for inate character traits and drives in the dogs. If the dog doesn't already have a retrieve and understand the basic concept, the compulsive retrieve is just plain mean dog training.

A retrieve is not necessary for the patrol dog, but a natural retriever and player is much more easily trained and much more enjoyable to work for the most part.