The concept of "innocent until proven guilty" is thrown out the window. Having a large amount of cash on you is not probable cause that you committed a crime, so how do they have the right to take it from you?
It is similar (on a much, much larger scale) to when you try to withdraw a large amount of cash from your bank account. The interrogation, and offers of a myriad of other options (in the bank's mind) which would be better (for them, although they try to convince for you), than withdrawing cash. Cashiers check, Money Order, wire, etc, are all strongly suggested. If you insist on the cash, the interrogation begins, and depending on the amount, forms are filled out to submit to the authorities because it is "suspicious activity".