That's a ball of crap in my opinion. "Redshirting", that is.
I'm sorry.
The grades that children are in are based on what abilities the average child has at each age and what they should be learning at that age to do well in their school careers. Kindergarten is for five-year-olds because its developmentally appropriate for most five year olds. It's NOT for six year olds because at six, most kids are ready for more challenging curriculum. Especially these kids, who have access to the best preschools and such. If your four-year-old is reading, what sense would it make to have a child who's been reading for two years in the same class with kids who don't know their letters?? In my opinion this has nothing to do with academics, its about having the "best" kid.
Working in an environment with there's heavy poverty and low socioeconomic status, some boys (and girls) simply aren't ready for kindergarten at 5 because of their home life, poor diet, lack of health care, or lack of social exposure. Pre-K is very, very important here and just about every child attends, for those reasons. The Pre-K program here is called VPI and is basically just kindergarten without the tests and learning standards, a training camp for kindergarten. By going to VPI when you're four lessens the chance that you won't be ready for kindergarten when you're five, which lessens the chance that you'll have the stigma of the "big slow kid" who's a year and half older than everyone in their class. I really can't see how they think this is going to be a good thing in the long run.
Like someone else said, I think this is just another case certain people gaining advantage over other people through unfair tactics and loopholes. Yes the six-year-old kindergarteners might be able to blurt out all the answers and win all the school yard races because they're bigger, but how is that really helpful? It tells the him that he should "always win" because he's being set up to win. What happens when he doesn't?