I'm not sure it's fair to call these parents unscrupulous. Some kids were leaving no matter what or regardless of who came to tryouts.
The ones who left properly know who they are. People do what's best for their kids and all families understood that. The Parents that came up to people at tryouts , that they hadn't said a word to all last season and "warned" them about everyone leaving and telling to come to tryouts? I think unscrupulous is a nice word to describe this behavior. I could think of a lot worse. Never thought or cared about the other kids they were effecting.
