The point of any Pigovian tax is to discourage its use ... <i>where that use is value-destructive</i> once you account for the externality. So it still doesn&#x27;t make sense to tax it by <i>more</i> than that value, which the plastic bag tax clearly does.<p>There&#x27;s a very common misconception that a) product X has a negative externality, therefore b) it must be value destructive on net, therefore c) no one should use it, therefore d) it&#x27;s good to slap arbitrarily high taxes on X.  But b-d) don&#x27;t necessarily follow from a).