Related

Here's another example of "failing to notice the subjectivity of what counts as social convention". Many people are annoyed by aggressive vegetarians, who think anyone who eats meat is a bad person, or by religious people who are actively trying to convert others. People often say that it's fine to be vegetarian or religious if that's what you like, but you shouldn't push your ideology to others and require them to act the same.

Compare this to saying that it's fine to refuse to send Jews to concentration camps, or to let people die in horrible ways when they could have been saved, but you shouldn't push your ideology to others and require them to act the same. I expect that would sound absurd to most of us. But if you accept a certain vegetarian point of view, then killing animals for food is exactly equivalent to the Holocaust. And if you accept a certain religious view saying that unconverted people will go to Hell for an eternity, then not trying to convert them is even worse than letting people die in horrible ways. To say that these groups shouldn't push their morality to others is to already push your own ideology - which says that decisions about what to eat and what to believe are just social conventions, while decisions about whether to kill humans and save lives are moral facts - on them.