A quarter of the US population is Evangelical Protestant, and that Catholics (and Orthodox) aren't actually Christians is a common belief among those 80 or so million people. It was certainly what I was raised to believe. Distinguishing mainline Protestants as believing Catholics (and Orthodox) are Christians is basically saying Protestants believe Catholics (and Orthodox) are Christians except for the tens of millions who don't.
I'm not anti-Christian. I'm anti-stupid.