This is a great question. For me, the steelman argument is the following: Without trade barriers, the local production economy is disadvantaged. Local companies have to comply with things like environmental and labor law regulations, making it more expensive to produce things. Other countries may not have these regulations. Therefore, by institution these rules without tariffs, the net effect is a reduction of environmental and social standards, as manufacturers move to locations where they do not exist.

A strong version of the policy would thus involve directly tying tariffs to environmental and labor laws of the producing country, thus equalizing the field with the local companies. Not only would this allow for fair international competition, it would also economically incentivise the global development of good standards.