The more finicky a distinction, the more fanatically people take sides over it. The Oxford comma (aka serial comma, series comma, etc.) is a case in point. Some people – often copy editors or writers – adopt it as a tribal badge and commit to it so completely that it becomes part of their identity. They become true believers.
Being a true believer means adhering to the faith: swearing, hand on stylebook, that the Oxford comma is the best option, end of story. ‘It eliminates ambiguity,’ they assert without qualification. Many claim to use it ‘religiously’, or they convey their devotion to it in analogous secular terms.
Either way, this is dogma, and like all dogma it masks a more complicated (and more interesting) truth.