While President Donald Trump’s attack on the US Federal Reserve Board and on a sitting governor, Lisa Cook, has elicited expressions of high alarm from economists and commentators, investors have responded with one big yawn.

This might seem like a dramatic disconnect. Actually, however, the two groups have good reasons for responding differently.

Commentators concerned with the stability and integrity of US monetary policy have every reason to be horrified by Trump’s attempt to “fire” Cook nearly 13 years before the expiry of her term. (Full disclosur...