I bolded the part you seem to be missing. Leaving games can get you banned as you're essentially screwing over 9 other people.
Correct. There is always a chance to turn it around and come back for a win no matter how bad the start. Although it may be a very low percentage chance. I was in a turn around game yesterday with Thayorn. I went Graves and he went Taric bottom. We were against a Sivir and a Sona. He took one ward only to start with. No big deal.
So we get to bottom lane and the other teams jungler, Lee sin, grabs red and tries a level 2 quick gank on us while we are level 1. It almost worked, but we got away. However, we did take some hits. After a couple of heals from Taric and some life stealing, I got back to nearly full. The problem was we were pushed up against our tower, which was stealing CS from me this early in the game. We were getting exps, but the tower shoots faster and harder to kill minions compared to me. Factor in the minions killing each other and it gets really hard to last hit with any reliability that early in the game. We needed to push out and push out safely somewhat from the tower, but sona was camping that side brush near the tower. Anytime we moved an inch from the tower she would pop out to Q us and power cord me. Then she would turn back into the bush and hide. If sivir could, she would toss out a Q to try to hit me as well. It was getting frustrating. Thayorn tells me over vent he is going to place a ward. I am thinking, "FINALLY! we gain some brush control back from sona." Instead he goes around the back side out towards river to ward the river bush. I see him going and ask him what the hell he's doing. He responds warding river, and I said, "Why the hell you doing that? We aren't in any position to be in danger from a gank from the river. We are getting out shit pushed in right on our tower from lack of brush control. Ward that brush by the tower instead!"
Of course he doesn't. Of course I can't last hit or push out now. I can always go back to base to heal fully, but then I risk letting sivir and sona gain to much in levels over me by doing so. I tell thayorn that we are now screwed because he did that bad ward placement. He says, "Just chill and last hit on the tower." I tell him I can't because the tower is taking too much CS from me to keep up with sivir. And both are capable of poking me on the tower. Which is exactly what ended up happening. We both kept getting poked by sivir and sona. Finally, when we could no longer keep up with healing enough from the pokes they then tower dove us. We both died while they both lived. This was at level 4 or so. Now w are really far behind as Sivir got both kills.
I was a bit pissed with Thayorn for that since it really was his fault. Not once did the ward he put in the river that first time be usable. We never once pushed out from the tower to need a ward in the river to prevent a gank. Instead we needed a ward in the brush to stop them from destroying us on the tower. So we respawn and this time I buy the ward. I place it in the brush. All of a sudden we now have brush control and sona can't hide in there to constantly poke us. We are able to push out from the tower without over extending. Or next problem came with Thayorn as Taric constantly trying to stun sivir. It would have been fine, but he never once tried to fake her out. So she spell shielded whenever he ran straight at her. He should have broke off to fake her into using her shield and then stunned her after the shield went down. But he would send his stun at her after sivir shielded herself everytime. Thayron doing this over and over was giving Sivir plenty more mana to constantly harass us back. It was getting annoying. Once they reached level 6 though they caught Thayorn trying to send a stun at sivir. Sona stunned Thayorn and me and we got destroyed before we could reach safety. This happened again as soon as we came back into lane for a second time, but the second time I managed to pick up one kill from Sona. So I was 1/3/0 at this point. Thayorn as Taric was now 0/3/0. Sivir was getting fed and had more CS than me. Things were looking bleak with a 6/0/0 Sivir in our lane.
One good thing finally happened next. We came back into lane after our last deaths, and Lee Sin tries to gank as soon as we arrive. The funny thing is that our jungler, Sejuani, comes into gank as well at that same moment. Making it a 3 on 3. For some reason they all focus Taric and Sejuani and leave me mostly alone. I managed to snag three kills while only Taric died and Sejuani barely got away. I was then able to farm a few minion waves to bring my CS up after that. Luckily Sivir went the stupid route and bought a blood thirster first. I got an IE first.
Soon after I was dominating. I ended up 17/7/21 for the game as Graves. I had IE, Tri-force, PD, Atmas, BT, and Zerker Greaves by the end of the game. I was sitting at 92% crit chance. My attack speed was 2.4 something after I would dash. Standing next to Taric who was running Aegis on top of his buff I would have 550 something AD. I was running crit damage runes as well so I had another +100% crit damage as well. I also had over 200 armor, 140 MR, and 2.6K health. I was pretty darn tanky thanks to graves passive, the aegis, and taric aura buff. I was literally 2-shotting anyone on the other team.
For awhile we WERE losing pretty badly. Mid and top weren't doing so hot. Or jungler was actually decent, but we were failing bottom as well because of a few early stupid decisions by thayorn. In the end though we came back with a vengeance.
That's why you don't leave games Malak. Wait for 20 and surrender. AFK or leaving is against the summoner code and will get you banned.