I've never ordered a "plain" burger, so I don't know, but I've ordered fast food burgers "only ketchup" or "only ketchup and pickles" and never had it arrive either (A) without a bun, or (B) with anything undesired.

At restaurants, I'll sometimes order "just meat and cheese," and once got asked "do you want a bun?" but I'm pretty sure it was waiter humor (like reading your order back with "and extra mayo" instead of "no mayo") and not an assumption that I was low-carbing it.

Like others have mentioned, I like the "you dress it yourself" approach to burgers; unlike at least one of the others here, I was not too impressed with the burgers at Fuddruckers.