Alright so I've been reported in the past for verbal abuse. I get the whole idea, but there is two things that can stop anyone from verbally harrassing you. One, turn on language filter. Two, Mute them. So pretty simple right? So why make it a reportable offense?
Second question. When is it okay to harass players? Like for example if someone is trolling or feeding? is it okay to get mad? 
inb4 salty.
Verbal abuse like any abuse is never justified. Despite it being possible to mute and language filter abusers, the whole point is that we shouldn't have to. It is a reportable offense because Riot along with many other league players want a civilized and likeable gaming community.
As for your second question, like I said above, it is never justified. You can get mad but that doesn't mean you have to flame them. No one is saying you can't get mad, but you can and will be held responsible for what actions you take when you do get mad. Being mad isn't a reasonable excuse for flaming behavior. There are ways to handle a situation without flaming and Riot wants to promote those ways as flaming tends to beget flaming and it mostly just escalates a situation.
Of course, as I don't work for or represent Riot, this is just speculation on my part.
Sure, flame all you want. Enjoy the ban.
Two wrongs don't make a right.
Trolls are trying to make you mad.
So no, getting mad at them is not useful.
Riot wants to promote all this but has an option for you to mute, and turn off language filter. Seems like they are okay with people using "vulgar" language. I mean I'm probably the minority but it honestly pisses me off when you waste my time. I work full time I would enjoy to play a game without a 12yr that gets butt hurt. If I've insulted someone they generally deserve it one way or another. I just don't see anyway of fixing the problem of idiots? I always think maybe if I guilt trip them into doing something they may change.
It is ok to be mad, frustrated, etc. But we're behind computer screens. It's not face to face. You have the option to just not say anything. Flaming always give the troll what they want to see, a reaction. So it's better to ignore and play the game.
Once again missing my point but okay.
