Jehovah's Witnesses...why is it that you 'disfellowship' people?

I have known people to be abandoned by their congregation, and maybe even disowned by their parents, if they committ a sin that they view as so great the person can no longer stay among them, lest the others be somehow 'tainted.' Why are these people turned away? Isn't it times like that that they need you the most? Shouldn't you trust your other congregation members to know better than to follow their example? In my church, sinners are welcome. Forgive me, if I sound condescending. I just want to understand.