Moving some money around without telling the government is only illegal because it's illegal.
To me, the difference is whether or not there is a victim. No victim, no wrongdoing.
If you knowingly helped a murderer dispose of a body, that’s wrong by nature right? Money laundering is the financial equivalent of that.
It's not necessarily disposing of a body, and punishing people for moving luggage around without asking questions is obviously absurd.
International airlines are a better analogy; should we just let people run weapons, drugs, even children in their luggage, with no mechanism for inspection?
If you don't tell the government you're moving resulting in taxes not being paid properly then you're damaging the government and by proxy, society. Atleast, that's what I think.
In cases where you're not tax avoiding, why does money laundering have to be a separate crime? It comes back to "Money laundering is just a cheat code for prosecutors to get easy points when they can't actually find real wrongdoing."
The first is what everyone can agree on is evil and the later is what we haven't agreed on (or not yet).
To quote from the page on malum in se; "The phrase is used to refer to conduct assessed as sinful or inherently wrong by nature, independent of regulations governing the conduct", which I don't think is a thing tbh.