I totally agree with that statement, but your understanding of righteousness is directly connected to obedience to the law not doing what God requires of us. The foremost lesson and example of "rigteousness" is of Abraham.
Gen 15:6And he believed in the LORD; and he counted it to him for righteousness.
Rom 4:2For if Abraham were justified by works, he hath [whereof] to glory; but not before God.
Rom 4:3For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness.
My point in posting those scriptures is we can find lessons (not commentary) teaching us that righteousness is independant of the law. Also proving Abraham did'nt have the law as a condition for righteousness.
Actually K4c, I was hoping to focus on your topic "Why".
Your OP "implies"
a) God uses the law to point out sin.
b) people teach that sin is not sin.
I had no plans to make this a sabbath day argument.
I was sensitve to what you claim "people teach" and responded to your word.
Let God's word be truth.