When the Prodigal Son asks for his inheritance, and his father gives it to him, is his father saying, "I want you to sin. I'm equipping you to sin. Because I know that you will eventually repent of your sin." When the Prodigal Son returns and the older brother is angry, does the father say to the older brother, "You were wrong to stay here and not indulge in sin. Your brother did the one thing necessary. He sinned. You didn't. Join the party next time and give yourself over to sin."
The father, who represents God in the parable, in giving us our inheritance is not equipping us to sin, but is equipping us with free will so that we may freely choose life. He sets before us two paths, and he tells us to choose the path of life; but he allows us to choose sin, the wages of which are death. And since sin does not always lead to remorse, we dare not go down the path of "death and destruction" (Deut. 30:15) with the intent of turning back once we've had our fill of pig slop and prostitutes. We never know when our gambit of planning a future turn to God may be lost; we never know when the path to death and destruction will simply take us there.
And the older brother is wrong not in refusing to waste his inheritance, but in refusing to rejoice in the repentance of his younger brother, once the younger brother comes to his senses and returns.
"But we had to celebrate and be glad, because this brother of yours was dead and is alive again; he was lost and is found." (Luke 15:32)
Now that's the common sense interpretation of the parable. To read this parable as an excuse for sin, indeed as an imperative TO SIN is phenomenally wrong. It's so wrong that nobody would come up with this on his or her own. It takes a bad preacher or a snake oil salesman to twist the word of God in such a way and to convince someone of it enough that they use it as evidence in a combox battle.
Or as St. Paul insists (in Romans 6) ...
1What shall we say, then? Shall we go on sinning so that grace may increase? 2By no means! We died to sin; how can we live in it any longer? ... What then? Shall we sin because we are not under law but under grace? By no means! 16Don’t you know that when you offer yourselves to someone to obey him as slaves, you are slaves to the one whom you obey—whether you are slaves to sin, which leads to death, or to obedience, which leads to righteousness?
But of course, St. Paul was a self-righteous prig and a Pharisee and a judgmental boor, wasn't he?