if at least Cain did that to the father, it would have been some punishment. "I will let this be with a warning" makes more sense if there was something in that warning to be escalated later.
just letting him get away with it like this sets a terrible precedent. and makes us look down on his brain.
He is supposed to get some punishment. Namely losing his seat in the Parliament. Plus some monetary compensation.
This is pretty weak because he keeps any other authority he has, in particular the one he has in this city which he obviously abused multiple times. And his criminal entreprises will not be impacted at all, along with any leverage this gives him on other authority figures.
Basically, this would be a minor setback to his ambitions, but not a significant punishment.
It is something, and Cain was ok with only this wrist slap, somehow.
Of course, being the stupid two-bit villain he is, he will not tolerate any setback at all and will immediately participate in an evil plot against Cain, bringing actual ruin on himself. Any non-braindead villain would lay low a few days, until Cain is gone, then resume (criminal) business as usual.
If Cain had not been 1. an S-rank adventurer
and 2. a foreign noble
and 3. engaged to multiple extremely high-rank daughters (3 that he already knows of, 2 more that would be easily revealed with minimum intelligence-gathering), maybe he would have some way to retaliate pretty soon with limited consequence. But he's going against someone who has extreme physical power (I include magic here), plus political and social influence, at an international level in this present case. There was even an official "warning: don't disrespect" notice.
The man is really looking for trouble.
(Of course, there is also point 4. Cain is a demi-god and acolyte of basically all the gods of their pantheon, but Malph has no way of knowing that so it doesn't count here. It will most likely count when it's time for retribution though.)