Of course all teams do this. They build a team around the talented kids they have. If the team directors perceive a weakness at a certain position they look to make a change there. In a try out some kids can have a great day and others can have a bad day. There are mistakes made. All of these teams in the various organizations are free to do what they want. All of these teams have dozens of kids trying out for what amounts to very few spots. Kids are going to get cut. Stop being so angry by what you think is unfair. Every kid at some point is going to fail. They will benefit from failure if you as a parent teach to not be angry and cry about lack of fairness. Instead you should teach your son grace and class and congratulate those that make the team. But most importantly teach them to learn from failure work harder and always get better. If remain so angry about what is perceived to be unfair and cannot get beyond you are doing your son a huge disservice. Talent and hard work eventually always end up on top | 
