That's a common misconception held by people who don't understand the nature of salvation. When a person becomes a Christian, the Bible tells us that he is "born again". His spirit, which once was dead, is made alive and joined with the Holy Spirit. He becomes a "new creation". His sin nature (that drive/propulsion to sin) is crucified, and is replaced with a new nature, created by Christ. This new nature changes the person'd desires. In his inner man/his perfect spirit, a Christian doesn't "want to" sin. That isn't to say that a Christian doesn't sin, because the flesh and spirit are often at war. However, when I do sin, I am convicted of it by the Holy Spirit. I hate sinning.
Are there people who claim to be Christians that seem to really enjoy sinning, don't care that they're sinning, and appear to use salvation as a license to sin? Yes, there are. But based on what the Bible says about the nature of salvation and "being born again", I would find cause to question in my heart whether these people are truly "born again".