Well if i can add my twopenneth worth!1
Sirstly, i don't agree that horses should be trained that way. What are they 'teaching' them anyway? How to evade the bit?
Secondly, They aren't acheiving anything, as the point of being 'on the bit' is to engage the hindquarters. None of those hindquarters were engaged. look at how poor some of the tracking up is.
Thirdly, it looks to me as though some of this comes from the riders holding on with their hands. (this may sound obvious, but i mean holding to keep
[their position) alook at the riders legs. they are not even in contact with the horse in some pictures and very tight in the knee in others. Perhaps this is to help them stay on their big moving warmbloods (thats a whole other debate;-))
I am sure the horses would suffer from headaches after this. As for the picture of the mare loose in the field, i think she is just starting to lower her head to 'snake' it from side to side. As they sat, "the camera never lies" but we all know it does. ;-) This photo was probably choosen because it shows a 'typical' andalusian posture.
Can i just say re bits, Pippa Funnell would only be allowed to use a snaffle at that level of dressage as it is only PSG up that you can use a double. It is a good photo to show the lack of tracking up is the other pic though.
What about the practice of dressage horses being 'too valuable' to go out in a field? How can a horse be too valuable to be a horse?

And how can top class racehorses (who are waaay more valuable) go out then?
Just to mention about Horatio Nelson, (horse injured and pts after Derby) the injury he sustained was not connected to the pre-race examination. Amongst his other injuries he sustained a twisted fetlock. This would indicate that Epsom's famous undulations took their toll. Very unfortunatley

especially as he was arguably the best bred horse in the world. Worth multi millions (much much more than any dressage horse;-))