Why A Porn Tax Is Unconstitutional

Following up on Friday's post on California's proposed 25% porn tax:  Eugene Volokh (UCLA) argues that such a tax would violate the First Amendment:

It seems to me the tax would likely be unconstitutional. Content-based taxes on the sale of First-Amendment-protected materials (and recall that the law targets not just unprotected and illegal obscenity, but also constitutionally protected pornography) are generally forbidden, see Arkansas Writers' Project v. Ragland (1987).