I think the situation is as simple as this:
Scenario #1: My kid won't shut up in class, causes a disturbance, gets sent to the office and I get a call at work...he knows he's in for a world of hurt when he gets home and he knows he should be.
Scenario #2: My kid won't shut up in class, causes a disturbance, and the teacher hurls something at his face or otherwise assaults him...he knows that I'll be in front of the Principal, Superintendant, School Board, etc as quick as possible to defend him and he knows he's still in a world of hurt when he gets home and he still knows he should be.
The latter here is what happened. We don't know how these parents disciplined their child for this, but we do know they are sticking up for him (as they should). Point is 12 year old kids, especially boys, will get into trouble. I would expect the adult to be more responsible and follow the guidelines established by the school district, which I'm sure doesn't include throwing a book at a student's face. When those established guidelines are broken I'm going to fight (not literally) as far as I can for my kid.