That's old news now. Apple had every legal right to do it. He was in violation of the Apple TOS, which clearly stated that you have to be 18 or older. This is done to protect Apple because he could easily steal Apple's work and use it however he wants, because minors can usually be void from any contract they sign.
Does it make it right? No. But Apple has to do whatever they can to protect themselves from future theft and/or lawsuits. It would even be possible for him to sue Apple when he turned 18 to take out whatever he contributed to Darwin, which in turn would be a huge mess. Apple is just a regular business, if they look over certain things, it could be held against them in the future.
What they *should* have is a program where select minors can join with parental signature/monitoring which the Parent would become responsible and could not later sue Apple or try to have his works removed.