Why isn't banning someone's champion ( if they deliberately told u not to ) classified as griefing?

Real life example:I was in my gold 5 promos, last game, I even got bugged by Riot( i had a free win which they didnt give, I had 2 wins already ) , I was in my last game and I get in champ select as MID, I ask my teammates not to ban Zed and one second later the TOP laner bans him, they h ad already picked Ryze and Yasuo , so I had to look up for their counters as I did not know them, there are 5 seconds left and I panic pick Diana , a champion that i never played before and didn't have any idea what her abilities were doing.I go into the game and get stomped by a yasuo main and a Sejuani that was camping me. My question is how come banning some1's champion when they deliberately told you not to ban him/her isn't classified as griefing if you have proof that they did it ?You ruin the player's experience and his game by putting him/her in an uncomfortable situation.