Why is whataboutism having a moment?

President Trump’s defense of his controversial pardon of former Arizona sheriff Joe Arpaio was precisely the defense that we’ve come to expect from Trump: What about these other pardons?

“[I]f you look at, as an example, President Clinton pardoned Marc Rich, who was charged with crimes going back decades, including illegally buying oil from Iran while it held 53 American hostages — wasn’t allowed to do that, selling to the enemies of the United States,” Trump said at a news conference on Monday. “He was pardoned after his wife donated hundreds of thousands of dollars to the Clintons.”

He went on with his list: pardons of “dangerous criminals,” of drug dealers, President Barack Obama’s commutation of the sentences of Chelsea Manning and Oscar López Rivera. What about them, he asked? Why is he being maligned when what Clinton and Obama did was so bad?