it is not always possible to tell if there is meat in a dish, but most decent people wouldn't try to pass off a vegetable quiche as vegetarian if the pastry contained lard.
The reasoning is very similar - not everyone is comfortable with eating meat products, and there are those for whom it would be a direct contravention of religious obligations.
The fact that people might not know without analysing the ingredients is really not the point, just as the fact that nobody might know they were being examined by a man without seeing under his skirt is a spurious and disingenuous argument that shows a distinct lack of respect for the agency of others.
A loveliness of ladybirds. So what are politicians?


