I think if someone, even the trainer in this video, had posted what actually happens on the first day of hold training, where, to put it mildly, the handler is effectively bullying the dog into taking hold while the dog is doing its utmost to resist, then the comments might have been less favourable or muted. You can see this on u tube and to quote the trainer, it's not pretty. I've used this technique in the past but I wouldn't wish to use it again because I no longer think it is any way to treat a dog, even if it does work. It was acceptable to me once, partly because public attitudes were different and partly because there were no effective alternatives, but there are now and I try to improve on how I train, especially if it means a more pleasant experience for the dog and me.
I do agree with Wullie that dogs might naturally carry stuff but they aren't hard wired to naturally bring the stuff they carry to hand, even though they often do the first time or two. In fact the main reason why these pressure hold techniques are resorted to is because of the handler's failing in making the retrieve to hand an aversive experience. So once again the dog is effectively being punished for the handlers mistakes yet taking the blame for it.
Naj
Just crossposting with Nickh and agreeing with him, in a way,

but I'd suggest that what some refer to as possessiveness is simply a learned behaviour inadvertently taught by the handler in effectively punishing the dog for delivering to hand. It's such a common problem because it's common to more traditional methods of teaching retrieving and people aren't moving on to newer, more effective techniques.