Silly antecedents

Dani Rodrik is a comedian:

Suppose you are a government that has some second-best policy instrument at your disposal (say tariffs) to correct a market failure which an industry is subject to. You recognize that the policy has some costs (rent-seeking, and so on), and you use your policies accordingly to maximize national welfare.

Hah! Hah!

Seriously. Why not just assume the distortion away by supposing firms and consumers have social welfare as their objectives? Do government buildings have magical selfishness-removing force fields surrounding them?