Confused about "He that is without sin cast the first stone"?

Okay, I'm not trying to knock the Bible at all, but I'm a little confused. So the Pharisees were trying to uphold the Law of God, which demanded the death penalty for the adulteress. They were doing exactly what God demanded for that particular sin. But by Jesus saying that only a sinless person can carry out God's commands for justice, he pretty much took away any possible way to enforce God's Law. So why would the all-knowing God, who the Bible claims never changes, enact the law and then scold the Pharisees for doing exactly what he asked? Because even though Jesus came and fufilled the law, he said he didn't abolish it so all penalties should still stand. This doesn't make much sense to me. Thanks in advance!