What's so hard about "plain"?

I'm a plain fast food person. Yes, burgers plain, chicken sandwiches plain, you name it.

I will add my own condiments, thank you very much.

But one thing that continually baffles me is how often I will shout into a drive thru speaker "PLAIN" and still get my sandwich or burger covered in crap.

I have now started the routine of parking at the window, receiving my meal, and unwrapping to check it... and usually sending it back through the window in exchange for a plain one.

I repeat, What's so hard about "plain"?