If it were as simple as someone saying "GG," there wouldn't be an issue. But tacking on "ez" at the end just to rub it in is not cool.
It reminds of an instance during little league baseball when I was a kid. The city brought in an out of town team to play my team because there were no other teams at our age group and not enough of us to divide us in to two teams. The out of town players were state ranked and competed nationally as well. After three innings, it was very clear that we were outmatched by only looking at the scoreboard. It was fourteen to nothing by the start of the fourth inning. At that point, the out of town team started being obnoxious. They were imitating chicken calls at our pitcher (he was new to the position and kept committing balk after balk after balk.) When they would get to home plate, they would all power slide a good five feet. There were other things as well. Oddly enough, none of the umpires or league organizers would ask them to stop. They all thought it was funny. At the fifth inning with a score of twenty two to nothing, the game was called. We were told to go shake their hands. Of course, the other team continued to make us feel bad by doing the "You're too slow" and moving their hands away. Needless to say, little league for our team ended that day. Many kids did not want to return and parents asked the league to return their entry fees. There still wasn't another team for us to play anyway.
You just were handed a humiliating loss in LoL. Not a single kill for your side or loss of any turret for your enemy. They trounce over you and always fall back when storming your Nexus. Finally, your surrender goes through and all you see in end game chat from the other team is "gg ez." Really? Did they HAVE to do that? The community has told Riot they don't like that kind of harassment and Riot punishes people for it since it's unsportsmanlike.
Go figure.