[WFTR]

Why, after wasting all of his money on prostitutes, he becomes so moral that he won't steal any of the food he's feeding to the pigs when nobody's looking, I'm not sure.

Technically, Jesus doesn't say that the prodigal son spent his money on prostitutes. Jesus said that he spent the money on "loose living" or some similar term depending on your translation. The older brother jumped to the conclusion of prostitutes in his anger and desire to put his brother in the worst possible light. Plenty of people live foolishly without actually hiring prostitutes or using drugs.

[Verginius Rufus]

St Luke says "living asotos" (meaning in an abandoned or profligate manner). It's true that it's only the older brother who mentions prostitutes, but he was probably on target. (What else would the money have gone for? Drugs? Gambling? Risky investments?).

Of course, we're only supposed to pay attention to the details that are actually mentioned--if something is "outside the story" it doesn't exist.