That only works if you know the little yellow dots can track you. Which nobody knew before it leaked (which was a while ago). But more importantly, its unlikely

That only works if you know the little yellow dots can track you. Which nobody knew before it leaked (which was a while ago). But more importantly, its unlikely a counterfeiter fills out warranty cards in the first place. More likely, the Secret Service uses the information to track down who the printer was sold to from the manufacturer, who that person sold it to, and so on down the line, until they get a name or description. When you're dealing with counterfeiting of U.S. currency, its a start, which they wouldn't have unless they had the little yellow dots. And since it's almost impossible to get rid of the dots, counterfeiters have to work a lot harder disguising where they got their printers.