Counterfeit pens are designed to detect starch on counterfeit paper. Real money isn't paper, but more akin to fabric. The assumption is that counterfeit paper will be designed with starched paper. If this were true, the *feel* of the bill would be so different from real money that the pen would likely not be needed.
The pens are, therefore, almost completely useless. Sure, they'll detect the bills made with cheap, starched paper. But if that were to ever occur, you wouldn't need the pen to tell the difference.