Because the point of the article is not to convince bitcoin advocates but to point out Krugman's intellectual contortions and dishonesty, when bitcoin possesses features that should appeal to him. The NSA has to walk a tightrope, because they want US government and other security-related communications to remain secure. They cannot recommend flawed encryption to their own. I have heard no mentions of SHA-256 being flawed, and the bitcoin software is open source, an unlikely honey trap. Use your own judgment.