What does Luke 15:25 mean?

The faithful, obedient older son is in the field, working hard for his father's farm. Some time ago, his younger brother had taken his inheritance and left (Luke 15:11–13). The older son would never shame his father in such a way. As the older brother draws near, he hears a celebration he wasn't invited to; the Greek root words suggest a band with dancing and singing. He had been working in the field. Why is there a party? Shouldn't he have been informed? He asks a servant what is going on. The servant explains that his shameful brother has returned, and their father has spared no expense in welcoming him home (Luke 15:14–24).

This older son may not know that his brother returned humbly, acknowledging his sin, and hoping only to be treated as a servant. He rightfully understands that this party is all his father's doing. Why is his father acting this way? Doesn't he know that by welcoming back the son who rebelled, he is slapping the faithful son on the face?

Jesus is telling this story to a group of Pharisees and specialists of the Mosaic law. They are incredulous that Jesus chooses to fellowship with tax collectors who take money from Jews for the Romans. They are offended that He eats meals with sinners who have walked away from the law God gave His people (Luke 15:1–2). They probably think that by eating with them, an important cultural practice, Jesus is endorsing or at least putting up with their treasonous lifestyle. Like the father, Jesus claims to be an honorable man. Why would He betray the Pharisees' law-abiding way of life for these lawless God-haters?