I think the thing is the majority of things the Murricans spread on bread are sweet things - the nearest they get to savoury is peanut butter (and even then they have that in sandwiches with jam... *pulls face*). So they're used to things you ladle onto the bread like an amateur plasterer. So maybe the trick is to get them started with things like devilled ham spread or anchovy spread as a sort of training regime for incredibly salty stuff, before introducing Vegemite into their orbit.