As followers of Jesus I expect us to have a higher calling, to change the world no less, perhaps bit by bit but nevertheless to be part of a chain of change. We cant do that if we give back the same that was given to us.
I think it probably also stems from a sense of humility. We all, I believe, born into the right circumstances and contexts could commit murder or lesser crimes, edged on by poverty, environment, necessity. We are all the same.
Turning the other cheek is not something I would do easily. But time and again in life I find it is the lesser of evils. It doesnt mean you let people beat you up again, walk over you or abuse you again, you just dont do it to them. And I think it makes you a better person and maybe at peace.
There are moving and courages stories of people who not only have turned the other cheek but have forgiven. They are rare God given, moving stories (see th Forgiveness Project for more on this), but I nor anyone else has the right to demand forgiveness. It is a rare and God given occurrance. Jesus simply asks us to be different, turn the other cheek, dont give an eye for an eye.