Apostasy was needed in the infancy stages of Islam and I understand why it remains today, but it's true, you can not enforce apostasy if someone leaves quietly. God is merciful, but there is certain lines that can't be passed. When you leave the religion and speak out against it, you're going against God's words and there's a serious punishment on earth as well as the after-life. People can't see the after-life, but they can see on earth. Therefore punishment is necessary here as well.
I can not stress it enough though. If you leave quietly you will not be put to death. Whoever is doing that is wrong.
Can't convert by force, either. That's wrong. What you might have heard is sparing of lives to those that convert during times of war, because it is forbidden to kill other muslims.