Avoiding animal products which may not meet criteria of your religion. Some people's religions allow the eating of meat or other animal products, but have restrictions on it. (As opposed to lifetimes' answer about religions that prohibit meat or animal products outright)

A workmate at a previous job eats vegetarian/vegan when she goes to restaurants because the meat (possibly dairy) may not be kosher.

Appeal over baby-killing butcher's $3000 compensation mentions a Queensland prisoner who chose vegetarian food because he was (incorrectly) told that halal food was not available.

I thought I heard about some case involving prisoners choosing vegetarian/vegan because they were offered food described as halal but were not confident the food was really halal, but all I can find is a mention of Guantanamo Bay's food being mainly vegetarian.

Avoiding animal products which may not meet criteria of your religion. Some people's religions allow the eating of meat or other animal products, but have restrictions on it. (As opposed to lifetimes' answer about religions that prohibit meat or animal products outright)

A workmate at a previous job eats vegetarian/vegan when she goes to restaurants because the meat (possibly dairy) may not be kosher.

Appeal over baby-killing butcher's $3000 compensation mentions a Queensland prisoner who chose vegetarian food because he was (incorrectly) told that halal food was not available.

I thought I heard about some case involving prisoners choosing vegetarian/vegan because they were offered food described as halal but were not confident the food was really halal, but all I can find is a mention of Guantanamo Bay's food being mainly vegetarian.