We don't have the whole story about how he obtained all these parents' addresses.  The school, while not explicitly saying it, seems to be of the opinion that he got parents' email addresses via official school records.  Could easily be the case-- suppose he was in charge of notifying the team of the first practice, and is given a school list of parents' email addresses.  He sends the notification to the list of emails from his personal email account.  Now, all those contacts are in his email program.  If he got his contacts like that, I think the school had a right to do what it did. 

Valuable lesson to be learned here either way-- NEVER SPAM YOUR INBOX CONATACT LIST!  Basic email etiquette.  There's no reason to believe that every person who has ever sent you an email wants to see your baby pictures, read your joke, or hear your political song.