Even a libertarian might eventually recognize that the refrain “internalize your externalities” is being used to exploit him: all anyone who wants to infringe on his liberty needs to do is utter the phrase and then make up an externality to suit.
You may not engage in homosexual activity because of the externality of God smiting the city and/or sending a hurricane.
You must be confined to your house and wear a mask because of the externality of grandma dying.
You may not own a gun because of the externality of children getting shot.
You must wear a headscarf because of the externality of … I dunno, Allah causing armageddon?
You may not eat hamburgers because of the externality of catastrophic climate collapse.
You may not use plastic straws because of the externality of sea turtles suffocating.
I think you could use a better example for “structure purist, content neutral”: that’s where carefully crafted deception (without being actually false) would go, and you undersell it by using a polite “white lie” as your central example.