> Why does being loving require mercy? He is doing a disservice...

Perhaps that's what Issachar's reservations were: caution that the analogy is only intended to illustrate the point rather than being the point. Let me try to explain why there is scarcely a "disservice" being done here.

Where the analogy differs is that a traffic violation is not the end-all, be-all, deciding final moment for the son. If the son had to pay the fine himself, it's not such a huge deal. But if we are punished for our sins directly, it's forever. Issues of learning, self-esteem, and responsibility are moot, null, and void -- you're dead for eternity, period. That's where mercy comes in. But if you look at how God handles actual traffic violations or other similarly minor incidents, God has, does, and will let us take the fall for it to teach us responsibility and the consequences of sin. He doesn't always let us take the fall, but sometimes he will, for whatever his reasons. But that's not the point here.