Did you know your color printer is probably spying on you? Most pages printed in color include "secret" yellow dots. Groups like EFF have "decoded" some of these, and found in every case that the dots include the serial number of the printer, and the time the document was printed.
Most likely, the US government secretly asked printer companies to include this tracking data in a misguided attempt to fight currency counterfeiting. But the problem is that anyone, not just the US government, who knows the secret code can use it against others. For example, the Chinese government could track dissidents who unknowingly print flyers on these printers, thinking they were anonymous.
The threat to anonymous free speech posed by these secret dots is too large to let the US government and the printer companies off the hook for their secret agreement. Yes, there may be risks to currency counterfeiting, but the solution is not to put anonymous free-speech at risk. The solution is to design currency to resist copying. In fact, most currency now cannot be copied due to watermarks, and very very tiny details. So even if these yellow dots made sense 10 years ago, the no longer help reduce currency counterfeiting, but they continue to put at risk those who need anonymous free-speech.