Really should be? Bullshit. Do you have kids?
No. Do you? And would you really do that, knowing it would get one of yours into a lot of unnecessary trouble, just because someone had a go at them?
It would be protecting your son if they were verbally abusive and you were verbally abusive back. It'd be protecting your son if they shoved him and you shoved them back. This wasn't protecting him, just lashing out because he got some stick, which I assume happens often.