Judges don't always stand for stuff like this, though I assume most of the people caught in this manner are leaned on until they plea bargain.
A while ago a family was flying out to visit their home in Africa and carrying a large amount of cash. When asked how much they had they said something like "About $4,000" and the agent they were talking to said that the estimate was fine and instructed them to write that down. When they got further in, though, another agent insisted that the money be counted and when it came out a bit higher than was written down used this as an excuse to seize the money. After a lengthy court battle, though, the family got the money back.
You probably know this already, but The Agitator has been doing a bunch of posts about seizing money recently. The vast majority are taken from minorities, and sometimes the cops threaten to take any children you are travelling with away from you if the money isn't forfeited on the spot. I'm probably getting some details wrong so check on him for details.
And then realise it's the people enforcing the law who are breaking it. Even if it isn't illegal, no chance they'd enforce that.