I saw some ears back, but I didn't really catch any that were back in a way that was aggressive or threatening to the trainer(s). With a rider, it appears that the horse is listening for cues ("flicking" its ears back in forth, such as the sorrel in the beginning doing a pivot). Same with the black horse still picture that was rearing; it doesn't seem to be doing so out of malice or threats.
In my [brief] experience, I know that back ears don't always mean you're about to be bit/kicked/challenged. At the stable I work at we have a yearling (lovingly named Thunder Wolf) who, once you approach him, will pin his ears and drop his head to "glare" at you; however, he will not bite (save for to maybe nibble on your coat sleeve or nip your boots), kick, or pull back at your touches.
I also noticed the horse bucking in the beginning of the video at the trainer's request: perhaps the ears being back are associated with the action normally depicted as aggressive among other horses? Simple habit?
I'm not against bits at all. I ride Creampuff, my mare, with a French link bit
and it takes a light touch to the reign and a vocal command (mostly "ho!" for stop) to get her to carry out my request. She receives this bit well! Otherwise, of the other horses, we commonly use the Western or Snaffle
bits. Some of them would reject this, so you have to have a louder hand than you would on Creampuff.
I suppose it would depend on the horse and rider. I know a lot of inexperienced riders use loud hands and no leg pressure, while more "seasoned" riders use more of their legs and a soft hand while maintaining an assertive disposition. Obviously I have no reason to not use a bit, even though I have seen some people yank on a poor horse's face; but if someone were to cause my horse's mouth to bleed with it, my perspective could very well change. (Many horses would then reject a bridle/bit due to fear of being injured again, as I've experienced before when a horse came in from the pasture with a cut lip.)
Regardless, I do rather like their method. The horses seem happier to work, whereas some livery horses I have seen will pin their ears and sometimes even buck when asked to work. The trainers' arms seem to be the horse's cue to perform (such as the one with the "bouncing" hand and the horse crow hopping alongside him).