Here's a recording of Steve Bierfeldt, a US citizen who tried to board a domestic airplane while carrying $4700 in cash, and was detained by the TSA and subjected to abusive language and threats when he said that he would only answer the TSA's inquiries ("Where do you work?" "Why are you carrying cash?") if he was required to by law. The TSA agents threatened to turn him over to the DEA. He was returning from a Ron Paul event in St Louis, MO, and worked for the campaign. The cash on his person arose from sales of t-shirts and stickers at the event.
The transport cops in the audio recording of his interrogation actually tell him if he's not guilty he has nothing to fear.
Exactly what security threat does cash pose to an airplane? Are suicide bombers wont to carry a lot of liquid capital in case they flub it and need to bribe their way out?
Cue clueless commenter who says, "Well what did he expect when he told the law enforcement person that he expected to be informed of his rights and legal obligations before he would answer his questions?" After all, constitutional liberties are only there to be admired, not exercised. In 3...2...1.