This is a perennial topic on HN, which is generally inhospitable to drug prohibition to begin with; it's possible to lay out the schematics of the counterargument:

* While there can't be any defense for the marketing of phenylephrine as a pseudoephedrine replacement, restrictions on pseudoephedrine are not irrational (that doesn't make them right, though I think they are).

* Pseudoephedrine by itself practically is methamphetamine, just in an unproductive chemical configuration. It is extraordinarily simple (though: not safe) to convert pseudoephedrine into meth.

* Pseudoephedrine is widely, practically universally available in the US without a prescription. It's a "behind the counter" drug, and, because of rampant abuse, access requires ID, like alcohol. Further, because the point of restricting pseudoephedrine is effectively a "rate limit" (to prevent people from acquiring enough Sudafed to make meth production practicable), Sudafed purchases are tracked.

* We've hashed out on HN the argument about whether that tracking results in spurious prosecutions. The one case I've seen us come up with, the arrest and prosecution of William Fousse, concerned someone who had a pseudoephedrine addiction (he was using it to come up from habitual alcohol benders).

* Restriction of pseudoephedrine does basically zero to staunch the flow of high-quality methamphetamine, which is produced at industrial scale with more sophisticated chemistry in Mexico and Asia.

* But restriction of pseudoephedrine might reduce the incidence of garage meth labs, which pose their own distinctive dangers to communities.

The argument in favor of continued pseudoephedrine restriction would be that the cost of the policy is relatively low (it inconveniences allergy sufferers, but most of those sufferers only marginally) vs. the public safety benefit (which is also probably low, but also probably nonzero).