I follow your reasoning, DCR, but keep in mind that this episode, like everything else ever produced since film was invented, plays to an audience on Earth, made up of people from Earth. Sometimes allowances have to be made in drama so the audience can more closely relate to the story.
Oh I agree, I write for a living so I'm familiar with the need of writing to the audience.
My actual point was more that the person wondering why paganism on alien worlds didn't draw the same reaction as Christianity was creating a false equivalence by comparing the generic paganism of the "Indians" on that one planet to the Christianity in "Bread and Circuses."
What adds to it, though, is that if I remember right there are mentions to the Classical Pantheon in "Bread and Circuses," as they named their cars after the old gods. That is a better comparison, but it's muddied by the fact that earlier that same season, in "Who Mourns for Adonais," it was established in canon that the Classical Pantheon were actually a race of alien explorers who early humans worshiped as gods, which could give an explanation for references to them on Magna Roma.
As for my own private "head canon," I just use the idea that there was an earthly connection at some point in the past.