Dej,
I get that too, but with me, they are always staring at or talking to my chest!
I've actually reached out, put a finger under the person's chin and lifted their head up and said "my mouth and eyes are up here" very quietly.
It's all I can do to hold my laughter at the color they turn, the sputtering noise they make and the way they hightail it out of my sight!
When I have been out with a family member or friend who is disabled and I am asked what they want to eat I always say nicely "I don't know, why don't you ask him/her?" or, if they are unable to respond, we have discussed it beforehand and then I answer for them.
I think in a lot of cases people just are afraid to make a mistake or embarrass the person, and they take the easy way out rather than asking someone who may not be able to answer for themselves.