Tithing, I think is a Christian practice. The Jews origianally sacrificed animals at the alter until their Temple was destroyed the second time and their religious practices changed to the Rabinincal form they have today; I don't believe "tithing" per se is pushed. I don't know about Islam. I believe most Eastern religions do not like the church to accumulate wealth and shun the concept of tithing, but that isn't to say they don't go on fund raisers of one sort or another for they have a mission to help the poor as well.
I think the formal idea of tithing, under the pretense of giving back to God, is simply to fund the church, plain and simple. This isn't a negative, for some of the money often does go to help the needy, but most of it goes to promote that particular brand of religion and pay for the infrastructure and the salaries of those who work in it; it simply a way for the church to survive.
What I find disinginuous about it is tying the giving to God. This "giving back" is interesting. In my view, it is all God's to start with, you don't possess it separate from God. It isn't like if you didn't want to give it back, God couldn't simply take it from you in one manner of another. Another way of looking at it is that God has "everything" already. By giving you something doesn't mean God has "less of everything" or when you give it back, God fills up the hole with it.
I think the Church simply needs to be upfront about it and say we need the money to survive.
As to greed, yep, it is involved in "some" cases. I drive around in the South and see these monserous mega-churches ... that is greed. On the flip side, I know many "religious-types" who are trying to buy their way into Heaven; that is greed to. I agree with a couple of the other commenters, that the true "giving" back is what you do personally to help others from your heart using the gift God gave you (and I don't mean your brand of religion either) and passing it on to others in need and not back to God.