President Trump likes to call for tariffs to punish foreigners who sell goods to Americans cheaply. Let’s assume, for the moment, that having American workers work fewer hours for the same standard of living is, indeed, bad; there is real injury caused by this action. If the goal of a policy is to punish the guilty, tariffs are the exact opposite of what one should do.
As Adam Smith says (Theory of Moral Sentiments, Page 155):
That the innocent, though they may have some connexion or dependency upon the guilty (which, perhaps, they themselves cannot help), should not, upon that account, suffer or be punished for the guilty, is one of the plainest and most obvious rules of justice.
The people buying the goods/services offered cheaply are not the guilty ones. The foreigners who are offering said products are. Therefore, to punish buyers through tariffs (which, outside extremely strong assumptions, fall at least partially on buyers) is a violation of “one of the plainest and most obvious rules of justice.”
Protectionism is unjust; plain and simple.