As a soon-to-be ex-president, Harry Truman was not lacking in lucrative offers from the private sector. Several corporations had allegedly offered him more than $100,000 to sit on corporate boards or to serve in symbolic positions when he left office. But Mr. Truman, whose reported income in 1954 was just $13,564.74 — about $120,000 today — steadfastly refused. Explaining his decision, he later wrote, “I could never lend myself to any transaction, however respectable, that would commercialize on the prestige and dignity of the office of the presidency.” Only by signing a contract with Life magazine for a goodly sum to write his memoirs was the former president able to exit from his financial difficulties.

Fast forward to 2017. This week it was announced that former President Barack Obama, whose net worth upon leaving office has been estimated at $12 million or more, agreed to give a speech for $400,000 — about seven times the median income of American families and precisely the amount of his annual salary as president — at a conference run by Cantor Fitzgerald, a Wall Street firm. A shortage of money can hardly explain Mr. Obama’s decision; according to press reports, he and Michelle Obama recently signed book contracts that may be worth as much as $60 million.

In accepting the fee, Mr. Obama joins a recent tradition of presidents monetizing their time in office by earning lucrative sums from speeches, corporate directorships, foreign corporations and other private interests. This tradition began with Gerald Ford, who accepted membership on corporate boards of companies such as 20th Century Fox and American Express after leaving office. Capitalizing on the presidency escalated decisively when Ronald Reagan accepted $2 million for a pair of speeches at Japan’s Fujisankei Communications Group. (In today’s dollars, about twice that amount.) And it reached its current-day apex with Bill and Hillary Clinton, who earned a combined $139 million from such undertakings, including $35 million from speeches to financial services, real estate and insurance companies.

Mr. Obama’s decision to accept the fee from Cantor Fitzgerald embodies an enormous attitudinal shift in the past six decades. When the financially strapped Mr. Truman turned down generous offers, he declined without hesitation, believing that it would violate his own sense of dignity as well as the dignity of the presidency. But no such normative constraints obtain in a society where the disruptive entrepreneur is the cultural hero, the public servant is held in low esteem, and inequality has risen to its highest levels since the 1920s. What was unbecoming in 1953 is now considered appropriate.