Feeding is situational. It only happens when the enemy is playing better than you. You don't always have a choice not to feed. They kill you, you don't suicide into them. If their jungler keeps camping you to towerdive you over and over, you have no choice. If your team is not making up for it, you'll lose the game. Simple as that: the enemy outplayed you.
Toxic, however is dispositional. You can always choose not to flame. It depends on the player's personality whether or not they do it. Of course it only happens in some situations, but it is inherently connected with the person's attitude.
If someone's bad at the game, they shouldn't ban them. They can just make them feed less by lowering their elo enough until they're at a ranking where they wouldn't. If you were up against a team of challengers, you'd either be feeding or be getting so hard you have less than half gold of everyone else. You can say "yeah but I'd just play safe and it wouldn't be as bad as feeding", that's true, but only if you KNEW in advance they were challenger. But you don't play like that every game. By playing extremely safe you're never going to achieve anything. If you just give up all farm early game, you'll be relying on your team to win for you every game. If you don't, you'll be feeding sometimes.
For example... I was playing Wukong the other day and laned against a hecarim. He cleared a jungle camp and tp'ed in lane after going B to come in lane on lvl 2 when I was still 1. He immediately went aggro on me thinking that he could get me out of lane with his lvl advantage. I just retreated while AAing him and he didn't notice that I had a lot more minions in the lane (cause I killed his first wave already) which caused him to lose over half his life. I soon got lvl 2 and immediately killed him. After that I jumped on him every time he left his turret range. Now you could say it was bad of him to let me jump on him, but honostly, he was just trying to farm and probably underestimated the range of my dash. After 3 kills, our Amumu jungler decided it would be funny to camp top so we towerdived him and staying his jungle to kill him before he even got into lane. He left laning phase 0/8/0. After that, he nearly made up for that in the teamfights where he disrupted a lot and got quite some assists. Now, he did perform very poorly in the laning phase. But is that because he's inherently bad or is it because he underestimated the match up and made a mistake? Granted that this was a normal game and he probably didn't play a lot of hecarim as he has only one ranked game with this guy (and he won it), he is a Diamond 4 so, no, he probably does not suck balls at this game. I watched his profile and he had a bad score in 2 of the last 30 ranked games he played. He often ends positive even when losing, but he went like 1/7 twice. During those games... Is he suddenly the worst player in history? That's probably what his tema would think if they had a toxic attitude.