It's really interesting how the right has embraced moral relativism on a case-by-case basis. Often it is a strategy to quarantine/localize ideas, so as to avoid the need to reconcile them to any broader worldview.

It's also a strategy for insulating ideas and events from history that they want to shelter from criticism, like criticizing slavery, theocracy, monarchism, etc. I've seen real cases in the wild where criticism of slavery was dismissed as "presentism", as inappropriately imposing present day moral values.