I can't speak for a nation, but those I have spoken to say they don't see an increased occurance of injury, the horses are backed at 2 and that's the way it is. There is also the 'they are quick to mature' argument (which is scientifically inaccurate in terms of their skeletal system) but because they
look fully developed they are considered mature. There is also, 'because we have always done it that way', which I think stemmed from the pioneer days, when people simply couldn't afford to raise a horse for 4/5 years before it started earning its keep.
I can speak to my own actions, Jess was backed just before she turned 3, in her 3yo summer she was lightly hacked, then just before she turned 4yo she went back for 1 months more intensive training and was schooling and competing regularly as a 4yo.
Frankly she was a demon horse as a 2yo, she had kicked me putting me on the floor several times, double barreling (repeatedly) me against fences/gates on more than one occasion, boxed me in the face (front foot square between the eyes) and was generally impossible to be around. She charged me when ever she was loose, she was fine as soon as I had hold of her, but getting close enough to get her was becoming more and more of an issue
She had been good for the 6 months prior, until her hormones kicked in.
I had planned to leave her another 9-12 months before backing but after talking it over with my trainer we concluded giving her brain something to think about and her learning some boundaries would be beneficial, and I didn't want to do that on the lunge/in the school because of her age, so decided to back her so she could start learning about the outside world a bit that year. It was the right choice for us at the time, and I don't regret it. I do not think any of her lameness issues have been caused by it because her X-rays were, to quote the vet "the best I have seen in a horse of her age and no signs of arthritis or any other boney changes". That is just one small part of her, but its a part prone to failure in many horses and she was no worse for wear.