This is just not true. You're probably thinking of vegans who aren't allowed to have cheese, milk, and eggs. Vegetarians can easily get B12 in their diets.
Vegetarians get B12 from the animal products created from animals which supplemented B12. Everyone is supplementing B12 regardless of their diet, it's just a matter of how many steps removed from the supplement that people seem to think makes a difference (it doesn't).
