In a idyllic utopia, yes, all forms of violence would never happen, let alone be unacceptable.
That isn't reality. In reality, where we all exist, s**t happens.
No, really, it does.
You are correct, noone should hit anyone ever. In the terms of a premeditated attack.
If it is something that happens in the heat of the moment, it should be judged accordingly, because who knows, the person who was hit COULD be at fault.
The other avenues are the grievance procedure and B&H procedure. A common complaint I've heard is that the procedures don't scale well to management issues, so a case may not be going the way it should go, or even how the member of staff thinks it should, leading to frustration towards the system. The system the managers represent. Frustration can lead quickly to anger, if there is a suitable catalyst, say a manager involved in the case ignores his request to forego lapsing today because his normal walk is busy that day, and makes him do it anyway, which means he might have to go over his time (in my made up scenario he is too proud to cut off, thinks his colleagues will think less of him), into the time he is ment to be picking his kids up from school, that he only see's once a week, after a recent separation with his partner.
See, add a little backstory, and it paints a different picture from the sterile 'mate punched lino, wat do?'
'Don't judge a book by its cover' fact.