Last year, Bryan Caplan wrote about what he called The Unbearable Arbitrariness Of Deploring:

Let's start with the latest scandal. People all over the country – indeed, the world – have recently discovered that many celebrities are habitual sexual harassers. Each new expose leads to public outrage and professional ostracism. Why does this confuse me? Because many celebrities do many comparably bad things other than sexual harassment, and virtually no one cares.

Suppose, for example, that a major celebrity is extremely emotionally abusive to all his subordinates. He screams at them all the time. He calls them the cruelest names he can devise. He habitually makes impossible demands. He threatens to fire them out of sheer sadistic pleasure. But the abuse is never sexual (or ethnic); the celebrity limits himself to attacking subordinates' intelligence, character, pride, and hope for the future. I daresay the average employee would far prefer to work for a boss who occasionally pressured them for a date. But if the tabloids ran a negative profile on the Asexual Boss from Hell, the public wouldn't get very mad and Hollywood almost certainly wouldn't ostracize the offender […]

Or to take a far more gruesome case: When the Syrian government last used poison gas, killing roughly a hundred people, the U.S. angrily deployed retaliatory bombers, to bipartisan acclaim. But when the Syrian government murdered vastly more with conventional weapons, the U.S. government and its citizenry barely peeped. The unbearable arbitrariness of deploring!

In the past, I've made similar observations about Jim Crow versus immigration laws, and My Lai versus Hiroshima. In each case, I can understand why people would have strong negative feelings about both evils. I can understand why people would have strong negative feelings about neither. I can understand why people would have strong negative feelings about the greater evil, but not the lesser evil. But I can't understand why people would have strong negative feelings about the lesser evil, but care little about the greater evil. Or why they would have strong negative feelings about one evil, but yawn in the face of a comparable evil.

He concludes people are just biased by dramatic stories and like jumping on bandwagons. Everyone else is getting upset about the chemical weapon attack, and people are sheep, so they join in.

I have a different theory: people get upset over the violation of already-settled bright-line norms, because this is the correct action if you want to use limited enforcement resources efficiently.