They are srs bsns, because if you don't take care of that shit problems will increase with it. One leecher indeed has little effect on your team, but ten WOULD have. So stop defending them.
And if we would follow your perverted logic, we weren't able to kick anyone, because it could be that the teamkill was NOT intended, that the last survivor wasn't just running away, but it was his way of playing, etc.
This is not the perfect world with absolute justice for everyone. You have to handle things as good as you can. 10 bans with 1 unjust decision is acceptable to me, to improve overall gameplay. Also don't forget, that if abuses get punished frequently, less people will abuse, so there will be less kicks/bans and so less undeserved punishments.