Because this law won't protect a single person against attack by a determined criminal. All it does is to increase the scope of State incursion into our private lives, create databases of purchases to be mined by all willing to issue a FOIA request or subpoena, and it likely won't do anything to deter a single crime. Moreover, one could simply use a different substance to commit a criminal act (e.g.: gasoline). There are already laws on the books that deal with misuse of substances for criminal activities, those should be enforced.
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