I didn't read the article or such, but in some cases you can be detained and even arrested for this. This goes back long before 9-11 and Homeland Security, etc..
If you don't declare the cash it can lead to quite a bit of inconvenience for you if discovered. Especially if you have trouble proving it is yours legitimately.
Also, some people are detained at the airport even after they declare the cash if they are leaving the country because other countries have varying amounts of cash that they let you bring into the country. So if you are leaving the US with $10,000 bound for CountryABC, they will detain you and let you know that CountryABC has a limit of $6000 that is allowed to be brought in and anything over can lead to a seizure of the entire $10,000.
Not sure what the circumstances were surrounding this as youtube is disabled here and the few articles I have seen remarked that the man acted suspicious and wouldn't answer questions. So not sure what his deal is all about.
