It shows they have no recourse but to actually malign the Gospel.
They refuse full atonement for sin, full forgiveness for sin, full redemption for sin, the full grace of God to forgive all sin through propitiating death of His Son.... that is basically what they are maligning when they assert a "license to sin"
Think of this from their perspective.. they cannot tolerate that they have nothing to add to their salvation ... in actually it is not about us it is about their problem with God and His Plan of Salvation.
This is about their SELF WORTH, their pride, their need to prove themselves... so we become the sinners, desiring to sin so their righteousness can be held up high.
That is a person who has denied the Gospel.... accuse me of a license to sin.... I make not apologies for the implication that we are fully saved by Grace and all sin was dealt with at the cross.