It's US law. Are there similar European laws?
The American attitude is: I’m not breaking the law until someone tells me to stop.
The American attitude is: the law is there to support the authorities.
Regardless of what the law says you don’t have any right to enter the US, regardless of what visa you hold (except perhaps a green card), so be friendly, look wealthy and important, and say the right things.
Every time I visited the US I noted a complete absence of common sense.
But only when entering the US I have been asked for example: Do intented to engage in illegal activity?
That's question seems to violate common sense: No one intending to do so would admit it.
Similarly to how you are supposed to pay taxes on income of illegal activities, if they find stacks of cash in your house it becomes easier to convict you.
They are weird laws but have their uses (good or bad)