Just like people, dogs and cats can show allergic symptoms when their immune systems begin to recognise certain everyday substances - or allergens - as dangerous. Even though these allergens are common in most environments and harmless to most animals, pets with allergies will have an extreme reaction to them.
I have seen this attitude before to allergies, that it is up to the allergic person to sort out what they eat, not for the restaurants to provide the information. That's fair enough in theory, but if the information isn't there for staff, and the person with an allergy asks about a particular dish, how is that member of staff supposed to answer?
It's a huge pain in the arse. And actually quite scary sometimes too. I have a fair few food allergies: nuts, which can set me off into an anaphylactic reaction, fish, makes my lips swell, hideous stomach cramps and vomiting, kiwi, bananas, mustard, raw eggs, milk - the list goes on. Not to mention hayfever, eczema, mild asthma.. yep, I was one of those kids.