Why do people with real allergies are assumed/accused of being vegans, when in fact have real allergies & medical treatment ?

My mother has a whole boat load of food items she has to stay away from, beef, lamb, pork, and chicken, squab, Turkey, fish and dairy products she has to stay away from but eggs she doesn't react to. Peanuts and lima beans, chillies and tree nuts, are not good for her either, she's also allergic to UVA/UVB solar rays.

We went to a restaurant where the owners understand my mother's extreme allergies and when we go there they have something prepared for her and ready to heat up.

My boyfriend and his parents went out to dinner, a while ago, but now his parents are spreading gossip that she's vegan and they are dead set against veganism 

Why do people with extreme allergies end up the subject of mean gossip like this?