It seems rather feeble minded to believe terrorists are going to obey any law, thus defeating any effort at taggants.

The gist is that many think a simple minded law is going to stop real criminals or terrorists with a simple understanding of chemistry. Black powder can be easily made by anyone from very common ingredients as can many other explosives.

This may come as a shock, but criminals and terrorists don't obtain weapons and explosives through normal channels because it increases their chance of detection. They aren't going to play the game by the rules.