Perhaps the “cultural/cafeteria Catholic” phenomenon has confused the issue for some people. There’s a non-trivial number of people who self-identify as “Catholic,” but pretty much reject basic tenets of Christianity (the creeds, moral teaching, etc.). There are Protestants like this as well, but they seem far likelier to disavow church affiliations altogether. Now, knowledgeable Catholics may understand that such a person isn’t Catholic in any meaningful sense. Protestant observers, though, aren’t really in a position to analyze his Catholic bona fides, but can certainly perceive that he’s no Christian. Hence some of the “Catholic <> Christianity” confusion.
I’ve since learned that they’re far–FAR–from representative, but nearly all the self-described Catholics I’ve met outside cyberspace are ignorant of (when not dismissive of) Nicene Christianity. Poor catechesis, I suppose. And of course, I’ve never lived anywhere with a significant Catholic community; nearly all the Catholics I’ve met are upper-middle class transient types who aren’t representative of the Church as a whole. Are such people more prone to this kind of irreligious faux-Catholicism? Dunno.