At the Port of Entry (or Exit), they do.
Ever wonder why searching luggage at a Customs station in the airport isn't a violation of the 4th Amendment?
If the diamonds were legal, the lady would've declared them. She didn't. Thus, the quite logical presumption that they are coming into the country for an unlawful purpose.
I'm not saying that they don't do it, I'm saying that it is another unwarrented government intrusion into our lives.
If the diamonds were legal, the lady would've declared them. She didn't
Legal? Again you're missing the point which is that the government doesn't have any business deciding what valuables I'm allowed to own, what I'm allowed to carry and which I'm not. Diamonds are a readily transportable form of wealth historically used by people fleeing repressive governments. Of course those selfsame government declare removing wealth from their countries illegal. My position is that the government doesn't have any business knowing what my assets are, what cash I'm carrying etc.