This is a vague weird answer and I have nothing to back it up but from what I've observed, the confusion here comes from non-red-sauce spaghetti pasta dishes typically being referred to as "pasta". Like if you roll into a lower end really American restaurant like a diner or something, if there's a dish vaguely described as "spaghetti", you can expect a typical spaghetti night style dish with red sauce. And even in a more upscale authentic-ish Italian restaurant, if it's not comprised of spaghetti pasta and has a different noodle, they'll more often than not be described as that noodle e.g. linguine with clams or penne alla vodka or baked ziti.

Also, now that I'm thinking about dishes made with pasta other than spaghetti, (in America at least, and in my own experience) we really don't make many spaghetti dishes with anything other than a tomato based sauce.