It's a mindset I've seen before. Belief in a single God, reverence for the Bible = "Christian", whether that's with a positive or a negative connotation - everyone who isn't a pagan or an atheist, or holding Eastern beliefs.

Jews and Muslims are a very small percentage of the US population, although the latter is growing. B'hai are even smaller, I believe. It's not really surprising that mindset seems to be default. It's just mildly annoying to be forgotten.